Do not center table headers by default

[dviererbe]

The furo base theme centers table headers. This looks weird in my opinion for tables with only a few columns. See for example:

The source, so that you can play around with the style yourself:

+-------------------+--------------------+
| lorem             |  ipsum             |
+===================+====================+
| dolor             | sit                |
+-------------------+--------------------+
| amet              | consectetur        |
+-------------------+--------------------+
| adipiscing        | eli                |
+-------------------+--------------------+
| sed               | do                 |
+-------------------+--------------------+

Centering table headers with this change is still possible, for example:

.. role:: center
   :class: align-center

+-------------------+--------------------+
| :center:`lorem`   | :center:`ipsum`    |
+===================+====================+
| dolor             | sit                |
+-------------------+--------------------+
| amet              | consectetur        |
+-------------------+--------------------+
| adipiscing        | eli                |
+-------------------+--------------------+
| sed               | do                 |
+-------------------+--------------------+

This uses the style of table.align-center defined in custom.css:

https://github.com/canonical/sphinx-docs-starter-pack/blob/1393e0b8b1fe2cd555bba38d80473fd67767a34d/.sphinx/_static/custom.css#L29-L32

[pmatulis]

I agree with having left-aligned table headers by default. Waiting a bit in case other TAs feel differently.

[pmatulis]

Good point @s-makin , and I feel that we should make the cheat-sheet addition in this PR.

[dviererbe]

Can this be merged?

[ru-fu]

I added two commits - one to fix a problem that it now wasn't possible anymore to centre a full table, and one to add examples to the cheat sheet. Are you okay with these additions?