[help] render user documentation once on startup

Currently we have two kinds of user documentation:

* the about page[1] which is written in HTML and part of the web
  application and can therefore link instance-specific pages
  (like e.g. the preferences) via Jinja variables

* the Sphinx documentation[2] which is written in reStructuredText
  and cannot link instance-specific pages since it doesn't know
  which instance the user is using

The plan is to integrate the user documentation currently in Sphinx
into the application, so that it can also link instance specific pages.
We also want to enable the user documentation to be translated.

This commit implements the first step in this endeavor (see #722).

[1]: searx/templates/__common__/about.html
[2]: docs/user/ (currently served at https://docs.searxng.org/user/)

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