DOC: Harmonize column selection to bracket notation
As [suggested by](https://medium.com/dunder-data/minimally-sufficient-pandas-a8e67f2a2428#46f9) @tdpetrou. If there is agreement about not _presenting_ the dot notation to access columns in the docu, please let me know and I'll then check the rest of the docu.

- [~] closes #xxxx
- [~] tests added / passed
- [~] passes `black pandas`
- [~] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] passes `git diff upstream/master -u -- "*.rst" | flake8-rst --diff`
- [~] whatsnew entry

+1 to generally using __getitem__ in the docs, other than the section describing dot access.

Can we think of a way to lint this, to ensure that we don’t regress on it in the future? I’m struggling to think of an easy way. A slightly harder way is to check for dot access of names that are in the columns with a python program run on the docs.   I'd be +1 for making this change comprehensively I will not be able to write python code to detect those occasions. Using `df(.?)\.([A-D])(\W)` for now and

- [x] checking the context.

Shall I also update old `whatsnew/*.rst` files? I think ignore old whatsnews for now - we usually don't go back and change them OK, I hope I didn't overlook any of these changes' context to explicitly mention "dot" notation/access/attribute. In order to add a `whatsnew` entry, shall I re-point this PR to [branch 0.25.x](https://github.com/pandas-dev/pandas/tree/0.25.x)? If yes, I would also add a `Documentation` section there, since I'm not sure into which of the pre-defined sections to put a summary of this change otherwise. Don’t need a whatsnew for doc changes. Also can just point to master - We can back port to 0.25.1 if needed (though not sure we would for this) No need for back port. OK :-) Anything else to do here, then besides fixing those CI errors? Maybe a note recommending [ bracket ] notation (or summarising its advantages)?

    The indexing operator ``[]`` provides a superset of the attribute operator ``.``,
    such as the ability to index column names using a variable, or column names
    that are the same as DataFrame methods, or contain spaces. Therefore, it is
    recommended and presented here preferably. I'll run the checks locally with `flake8-rst doc/source/ --filename="*.rst" --format="default"` to debug all these issues. Thanks @katrinleinweber! You're welcome! Thanks to you though, for agreeing with this & for merging :-)