DOC: Use container directives instead of raw html for the doc "cards"

[stijnvanhoey]

• closes #xxxx
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

The PR is a follow up of #31156 and #31148 translating the raw html to docutils/sphinx directives, making it more readable.

Furthermore, it fixes the redundant css in pandas.css versus getting_started.css by removing the install-card class and merging it with the existing intro-card class.

Note: The setup required an adjustment to the theme to overcome the usage of the container class by both docutils and bootstrap.

[simonjayhawkins]

@stijnvanhoey can you merge master to get the ci green.

[WillAyd]

I think looks good but @jorisvandenbossche

Is this directive provided by sphinx or someone else?

[jorisvandenbossche]

Yes, it's even from base docutils / restructuredtext.

But, this still depends on a change in the sphinx theme to support the way it is styled (a conflict in a css class needs to be solved); so this needs to wait on that (pydata/pydata-sphinx-theme#92)

[datapythonista]

Looks like both this PR and the one is depending on have conflicts. Are we still moving forward with this?

[jorisvandenbossche]

Yes, I think we still want to forward with this. The linked PR mainly conflicts due to the rename, will try to move forward that PR.

[chrisjsewell]

Hey guys, just an FYI, that I took your good idea here and turned it into an extension: https://github.com/executablebooks/sphinx-panels

(in this extension I also provide a "fix" for pydata/pydata-sphinx-theme#92)

[jorisvandenbossche]

First try to convert some of the cards to use sphinx-panels -> #34065 (still needs to be further updated, though). So let's close this PR in favor of that one.