DOC: Remove doc spellcheck
The spellcheck system was implemented, but with dependencies (i.e. `enchant`) that are no longer maintained, and there is no reasonable way to set them up. This means that can't be added to the CI, and can't be even set up into our local environments with a decent amount of work installing dependencies manually (I didn't succeed).

The PR that was trying to finish the work on the spellcheck, #21402, was finally discarded, but part of the stuff is still in sphinx, just generating a warning when trying to get the sphinx extension.

This PR removes it.
Hello @datapythonista! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`doc/make.py`](https://github.com/datapythonista/pandas/blob/b89fcc50f05496589048b817cffc8be2a09cf391/doc/make.py) !

 - There are no PEP8 issues in the file [`doc/source/conf.py`](https://github.com/datapythonista/pandas/blob/b89fcc50f05496589048b817cffc8be2a09cf391/doc/source/conf.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=h1) Report
> Merging [#24287](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7b0fa8e94e22aa4a491fa574bc56f8ca791f7608?src=pr&el=desc) will **decrease** coverage by `49.21%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24287/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #24287       +/-   ##
===========================================
- Coverage   92.22%      43%   -49.22%     
===========================================
  Files         162      162               
  Lines       51824    51824               
===========================================
- Hits        47795    22289    -25506     
- Misses       4029    29535    +25506
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `43% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-98.65%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-95.46%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.17%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.15%)` | :arrow_down: |
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `10.44% <0%> (-89.56%)` | :arrow_down: |
| ... and [119 more](https://codecov.io/gh/pandas-dev/pandas/pull/24287/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=footer). Last update [7b0fa8e...992a4ad](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=h1) Report
> Merging [#24287](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7b0fa8e94e22aa4a491fa574bc56f8ca791f7608?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24287/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24287   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         162      162           
  Lines       51824    51824           
=======================================
  Hits        47795    47795           
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=footer). Last update [7b0fa8e...65b7493](https://codecov.io/gh/pandas-dev/pandas/pull/24287?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! can you do a followup to remove sphinxcontrib-spelling from any of the ci & environment.yml & also remove the spelling section from the contributing guide. thanks. Thanks for pointing out @jreback, I totally forgot about it (and I double checked, and there were also some code left in `doc/make.py` I wasn't aware of).

I opened this follow up: #24299