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.
b4eda56fb0