DOC: Add docstring for DataFrame.T property
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.



**Before this change:**
The `T` property of a DataFrame is named in the docs but is not linked to any documentation:

(Screenshot below taken from https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.html)

<img width="1346" alt="image" src="https://user-images.githubusercontent.com/122238526/212579272-bc583d1a-bd0d-471e-972f-c6d10d81667e.png">



**After this change**
The `T` property is linked to documentation (it now appears as the top property alphabetically, I guess because it's a capital 'T').

<img width="1258" alt="image" src="https://user-images.githubusercontent.com/122238526/212579535-b4f9c4d7-2bfb-49af-bb3b-1e3f0e675df0.png">


And if you click on it you see this new page.  As you can see, it links through to the existing `DataFrame.transpose` docs. I put in one simple example here.
<img width="1277" alt="image" src="https://user-images.githubusercontent.com/122238526/212579665-cdca6ebd-9a99-4d16-ace6-979ad85a7dbc.png">





Thanks @smij720 