DOC: add doc on ExtensionArray and extending pandas
- Added a new page on extending pandas
- Added some prose on extension arrays / extension dtypes
- Moved subclassing there
- Added a section to ecosystem.rst about libraries providing extension types and accessors.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19936?src=pr&el=h1) Report
> Merging [#19936](https://codecov.io/gh/pandas-dev/pandas/pull/19936?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6ef4be3f8f269f147b5abedecf7da6f19af305d3?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19936/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19936?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19936      +/-   ##
==========================================
- Coverage   91.69%   91.66%   -0.04%     
==========================================
  Files         150      150              
  Lines       48981    49026      +45     
==========================================
+ Hits        44913    44939      +26     
- Misses       4068     4087      +19
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.83% <ø> (-0.03%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19936?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/accessor.py](https://codecov.io/gh/pandas-dev/pandas/pull/19936/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWNjZXNzb3IucHk=) | `98.7% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19936/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/19936/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.14% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19936/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.88% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19936?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/19936?src=pr&el=footer). Last update [6ef4be3...87aca05](https://codecov.io/gh/pandas-dev/pandas/pull/19936?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Doc build finished. Merging in a bit. Thanks!