Python API documentation requires the ability to import modules. So, we
set up a virtualenv in our Sphinx environment so module loading works.
This solution isn't perfect: a number of modules fail to import when run
under sphinx-build.
Previously, code for staging the Sphinx documentation from moz.build
metadata lived in a mach command and in the moztreedocs module. This
patch moves the invocation to the Sphinx extension.
When the code is part of the Sphinx extension, it will run when executed
with sphinx-build. This is a prerequisite to getting RTD working, since
sphinx-build is the only supported entrypoint for generating
documentation there.
With this patch, we can now invoke sphinx-build to build the
documentation. The `mach build-docs` command is no longer needed.
Read the Docs has a lovely Sphinx theme that beats the pants off the
built-in and default theme. And since it looks like MDN's Sphinx theme
is dead in the water, this gets us a nice UI win until the MDN theme
comes back from the dead.
