The docs should present mpl_toolkits as more or less regular modules,
that just happen to live in a different toplevel package for historical
reasons. So just concatenate them to the end of the main module list.
(This is particularly in preparation of the new website redesign, where
the separate "Extra Toolkits" grouping is really too conspicuous on the
front page.)

Remove the toolkits/index page (documenting mplot3d and
axis_grid/axisartist ("ag/aa") together doesn't make much sense except
for this historical grouping), merging the mplot3d intro blurb together
with the one previously at mplot3d/index.rst; the ag/aa blurbs were
just included from the ag/aa API docs themselves and therefore don't
need to be kept.

The mplot3d intro blurb itself can go to the top of the mplot3d API
docs, rather than needing a separate page.

PR Summary

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).