This adds documentation for how to build the docs locally.

Previously the `make docs` command required you to have Python 3 and tox
installed system-wide, neither of which are required for any of our
other projects anymore, and this wasn't documented anywhere.

This PR changes `make docs` to use pyenv to install Python 3 and tox, as
all our other Python projects do, and adds a Building the Docs Locally
page to the docs with instructions on how to install pyenv and build the
docs locally.

pyenv and tox are _not_ added to the general client dev install
instructions since they're only needed to build the docs.