docs, user: Fix faq menu rendering glitch
As the toctree for the faq was included multiple times in index file, the associated menu entry in the sidebar was not properly rendered.
In order to workaround the issue, ensure faq toctree is included once in the index file and use references to faq sections instead in documentation main page.
Also as faq subsection entries are no longer included in the sidebar menu, display a local toctree at the top of the faq page.
Closes #3657 (closed)
Migrated from D6489 (view on Phabricator)