Add green, red and yellow side borders for code examples

[hugovk]

Like python/peps#3567 and python/devguide#1237, re: python/docs-community#22.

This will allow us to add borders on the left side of code examples to flag if they're "good", "bad", or somewhere in between.

For example:

Do this:

.. code-block::
   :class: good

   a = 1

Don't do this:

.. code-block::
   :class: bad

    a =

Take care doing this:

.. code-block::
   :class: maybe

    a = 2

Gives:

And this will be useful for the upcoming free-threaded guidance coming to the docs.

cc @lysnikolaou

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📚 Documentation preview 📚: https://python-docs-theme-previews--285.org.readthedocs.build/

[StanFromIreland]

LGTM

[lysnikolaou]

LGTM! Thanks @hugovk!