[mod] replace /help by /info pages and include pages in project docs

This patch implements a bolierplate to share content from info-pages of the
SearXNG instance (URL /info) with the project documentation (path /docs/user).

The info pages are using Markdown (CommonMark), to include them in the project
documentation (reST) the myst-parser [1] is used in the Sphinx-doc build chain.

If base_url is known (defined in settings.yml) links to the instance are also
inserted into the project documentation::

    searxng_extra/docs_prebuild

[1] https://www.sphinx-doc.org/en/master/usage/markdown.html

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

requirements-dev.txt

@@ -14,6 +14,7 @@ sphinx-jinja==1.4.0
 sphinx-tabs==3.2.0
 sphinxcontrib-programoutput==0.17
 sphinx-autobuild==2021.3.14
+myst-parser==0.17.0
 linuxdoc==20211220
 aiounittest==1.4.1
 yamllint==1.26.3