move generic docs stuff to `scverse-doc`?

OK, so scverse-doc now almost exists. Should we move generic docs stuff there? E.g. some or all of what was added in #60 is not related to any runtime feature scverse-misc offers.

I wonder how we should handle it:

should scverse-doc depend on scverse-misc[sphinx] and load its extension? Then people could choose:

• dependency-groups.docs = [ "scverse-doc" ] and extensions = ["scverse_doc"] gives them all scverse-doc features and all the scverse-misc features they activate in their project.dependencies
• dependency-groups.docs = [ "scverse-doc" ] and extensions = ["scverse_doc.foo"] gives them only the subextensions they want (like scanpydoc)
• dependency-groups.docs = [ "scverse-misc" ] and extensions = ["scverse_misc.sphinx_ext"] give them just docs support for the runtime features, no additional docs features

but we’ve already deviated from that with #60 …

Start by reading the changes in #60 and reviewing the existing scverse-misc documentation support alongside scverse-doc. Compare the proposed dependency and extension-loading options, then define where generic documentation features should live and how users should activate them.