DOC: make canonical version stable

[jklymak]

PR Summary

Depends on matplotlib/matplotlib.github.com#47

Makes matplotlib.org/stable the canonical root.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[jklymak]

Added an update of release_guide.rst to explain how to maintain this upon release...

[jklymak]

I think this is right for now.

It would be nice if "canonical" changed for the newest version of an obsolete page. However, for now I think its best for search engines to try and find it at /stable/

[jklymak]

If matplotlib/matplotlib.github.com#49 goes in, this will need to be slightly modified

[jklymak]

This has been modified to take into account matplotlib/matplotlib.github.com#49

[QuLogic]

I'm not an SEO expert, but this sounds good.