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.
In order for Sphinx documentation to work with Read The Docs, we need
the code for scanning the build config for Sphinx documentation to live
in an importable module. This patch moves some code from the
|build-docs| mach command into an importable module.
