Test documentation builds

[kurtmckee]

ReadTheDocs hasn't built the documentation in over a year.

To help ensure that the documentation can build cleanly, this PR introduces a new tox build environment, docs, which tests that the documentation can build.

It also fixes a single warning issued by the new doc tests:

WARNING: html_static_path entry '_static' does not exist

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Note

While working on this, it became clear that tox.ini is not maintained (Python 2.7 and 3.6 are the only defined Python versions to test). This can be addressed in a separate PR.

Also, it is strongly recommended that doc/requirements.txt pin exact dependency versions.
This PR does not address build reproducibility; it simply introduces testing of the documentation builds.

[acarapetis]

Thanks. You're right that the tox config is no longer maintained - I've just been relying on the circleci test matrix instead, which does the job fine.

@djrobstep would be good to get the webhook configured to autobuild rtd.