DOC: Add docstring for DataFrame.T property

[smij720]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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)

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').

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.

[luke396]

There should be only one blank line here.

And it seems more appropriate that the order is Returns, See also, Examples.

[smij720]

Thank you - agreed. I have made those changes.

(for anyone else reading this - the reordering edit in response to this feedback means that the doc sections in the code now match the order in which the docs are rendered, which makes more sense)

[mroeschke]

Thanks @smij720