enable syntax highlighting in include directive

[drammock]

WIP. Makes it so that the content of .. include:: directives (handled by docutils) with the :code: option gets lexed in the same way that Sphinx-handled .. code-block:: directives are lexed (using short classnames like "ow" instead of long ones like "operator word"). Relies on the presence of a docutils.conf file; so far I've got it working for tox run -e docs-live but haven't looked into the deployed docs / CIs to make sure it gets picked up there.

xrefs:

• using include directive with :code: option doesn't highlight the code sphinx-doc/sphinx#11185
• DOCS: admonition customization #1155

[drammock]

it worked on the CI doc build too: https://pydata-sphinx-theme--2222.org.readthedocs.build/en/2222/user_guide/extending.html

note how the CSS samples are now syntax-highlighted, as are the rST samples in the tabbed boxes. The markdown samples (the other half of the tabbed boxes) aren't highlighted for (I think) two reasons:

1. they're in a .. code-block:: not an .. include:: so their HTML nodes (and classes) are different
2. because they're bits of MyST-specific markup, they're not actually lexed very well as markdown, and AFAICT there isn't a myst-specific lexer built into Pygments (though I think one exists, based on a quick scan of the source code of e.g. this page)