[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>

searx/templates/simple/info.html

@@ -0,0 +1,14 @@
+{% extends 'simple/page_with_header.html' %}
+{% block title %}{{ active_page.title }} - {% endblock %}
+{% block content %}
+<ul class="tabs">
+{% for pagename, page in all_pages('en') %}
+  <li>
+    <a href="{{pagename}}" {% if pagename == active_pagename %}class="active"{% endif %}>{{page.title}}</a>
+  </li>
+{% endfor %}
+</ul>
+<div class="info-page {{pagename}}">
+  {{- active_page.html | safe -}}
+</div>
+{% endblock %}