[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>

docs/user/index.rst

@@ -2,8 +2,14 @@
 User documentation
 ==================
 
-.. toctree::
-   :maxdepth: 2
-   :caption: Contents
+.. contents:: Contents
+   :depth: 3
+   :local:
+   :backlinks: entry
+
+
+.. _search-syntax:
+
+.. include:: search-syntax.md
+   :parser: myst_parser.sphinx_
 
-   search_syntax