DEPS: Adding missing doc dependencies to environment.yml
In #26648 we detected that `environment.yml` didn't contain some of the dependencies required to build the documentation. `environment.yml` is what we use to build the documentation locally. Adding them here.

CC: @jorisvandenbossche 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=h1) Report
> Merging [#26657](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/01d97d48b08c546a46b91c27a5886f52b46f22c2?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26657/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26657      +/-   ##
==========================================
- Coverage   91.87%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50683    50683              
==========================================
- Hits        46567    46564       -3     
- Misses       4116     4119       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <ø> (ø)` | :arrow_up: |
| #single | `41.81% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26657/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26657/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.91% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26657?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/26657?src=pr&el=footer). Last update [01d97d4...4d0d731](https://codecov.io/gh/pandas-dev/pandas/pull/26657?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/26657?src=pr&el=h1) Report
> Merging [#26657](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/01d97d48b08c546a46b91c27a5886f52b46f22c2?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26657/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26657      +/-   ##
==========================================
- Coverage   91.87%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50683    50683              
==========================================
- Hits        46567    46564       -3     
- Misses       4116     4119       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <ø> (ø)` | :arrow_up: |
| #single | `41.81% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26657/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26657/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26657/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.91% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26657?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/26657?src=pr&el=footer). Last update [01d97d4...4d0d731](https://codecov.io/gh/pandas-dev/pandas/pull/26657?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hmm this might cause some issues with xlrd throwing pending deprecation warnings as defusedxml seems to be getting installed as a dependency. 

Seeing that locally at least let's see if it comes up in CI The CI is not using this file. May be you can open a issue with what you see> I didn't see any new warning in the documentation build, didn't run the tests with this environment after the change. For sure. I think it might be manifested in #22682 though if I see something more concrete will open an issue