Consider including docs for each released version on GH pages

[c24t]

We rebuild the API docs at https://open-telemetry.github.io/opentelemetry-python/index.html on each change to master. One benefit of hosting on readthedocs.org (#452) is that we can host docs for multiple versions of the library at once. E.g. for each release and the "latest" version, which is built continuously from master.

It looks like it's possible to do the same for the docs on GH pages with https://github.com/sphinx-contrib/sphinxcontrib-versioning.

[lzchen]

Closing. We are already hosting on readthedocs.