DOC: Add an option to disable Google Analytics.

[QuLogic]

PR Summary

Fixes #5823.

PR Checklist

• [n/a] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [n/a] 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 pydocstyle<4 and run flake8 --docstring-convention=all).
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[dstansby]

Is there a way to more intelligently only add the google analytics code for pages served on matplotlib.org? If not this seems like a good solution.

[timhoffm]

Is there a way to more intelligently only add the google analytics code for pages served on matplotlib.org? If not this seems like a good solution.

Can't we configure the build to not include analytics by default, and only optionally enable it via a parameter sphinx-build -D or sphinx-build -A.
Then, only set that parameter for the release doc build.

[QuLogic]

Ah, I didn't see that in Sphinx; that seems a good way to do it.

[timhoffm]

Waiting for another review since @tacaswell's approval was for another solution.