• Rewrite rc_context using contextmanager, which removes the need for
  saving state.

• When__exit__ing rc_context, skip rcParam validation, as the items
  have already been validated originally.

The rationale for not revalidating (beyond a minor gain in speed) is
that it may make sense to smuggle seemingly "invalid" values into the
rcParams (by callingdict.__setitem__), and we do not want them to
cause an error when exiting a rc_context (of course, they may or may not
cause an error at the backend level, but that is the responsibility of
whoever set this "invalid" value).

For example, the rcparamlines.antialiased is declared as a boolean,
but the mpl_cairo backend actually also recognizes any of cairo's
internal antialiasing enum values (NONE, FAST, GOOD, BEST, etc.).

(The rst docs needed to be slightly updated because .. autoclass::
rc_context won't work anymore, so I also took advantage of that to
reorder all the rc-related functions slightly more logically.)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way