DOC: Adding header to the rst files that don't have it
When using the new style for the pandas documentation, we'll be using the `{{ header }}` context to set the `:tocdepth:` of all pages (sphinx does not have a setting to set all the `:tocdepth:` at once).

In #24086 we added to most `.rst` files the header, that it's also used to import pandas and numpy, and set some settings. Adding it to the pages that don't have it yet.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24191?src=pr&el=h1) Report
> Merging [#24191](https://codecov.io/gh/pandas-dev/pandas/pull/24191?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/77278ba8158457f142e6bba19e6d53a66a3f1322?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/24191/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24191?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24191      +/-   ##
==========================================
- Coverage   92.21%   92.21%   -0.01%     
==========================================
  Files         162      162              
  Lines       51723    51723              
==========================================
- Hits        47695    47694       -1     
- Misses       4028     4029       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43.03% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24191?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/24191/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `92.61% <0%> (-0.48%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24191/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.51% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24191?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/24191?src=pr&el=footer). Last update [77278ba...e794715](https://codecov.io/gh/pandas-dev/pandas/pull/24191?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/24191?src=pr&el=h1) Report
> Merging [#24191](https://codecov.io/gh/pandas-dev/pandas/pull/24191?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/138a59a9a06c5f0c31bb853113f7dea90b5a556d?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/24191/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24191?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24191   +/-   ##
=======================================
  Coverage   92.21%   92.21%           
=======================================
  Files         162      162           
  Lines       51723    51723           
=======================================
  Hits        47695    47695           
  Misses       4028     4028
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43.03% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24191?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/24191?src=pr&el=footer). Last update [138a59a...edae762](https://codecov.io/gh/pandas-dev/pandas/pull/24191?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @datapythonista! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`scripts/validate_docstrings.py`](https://github.com/datapythonista/pandas/blob/edae762fc3baa4c3f901ada2233236d30afef4e0/scripts/validate_docstrings.py) !

 Didn't think about it. It could make sense in pages with no examples (e.g. `api.rst`) where we don't need to imports. But I think it'd add some extra complexity, having two jinja variables in almost each file, and also things like `.. currentmodule::` would be ambiguous to know in which one should go.

We can always go back to it. But for now, besides making the imports simpler and more consistent, it allows me to set the `:tocdepth:` in all documents at once, and have reasonable sidebars in the new design (still wip of course): https://pandas-dev.github.io/pandas-sphinx-theme/pr-datapythonista_base/