Sphinx was displaying "WARNING: document isn't included in any toctree"
for files we just don't want in the TOC. We can hide them from the index,
but the rtd theme defaults to display hidden index entries in the nav bar.
This commit excludes these files from such warnings, and patches layout.html
so that hidden index entries stay hidden from the navigation sidebar.
This commit also moves password-strength.md under docs/production and
adds it as a hidden entry in production/index.rst.
Fixes#7417.
David Rosa Tamsen