[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/oscar/navbar.html

@@ -3,7 +3,7 @@
         <a href="{{ url_for('index') }}">{{ instance_name }}</a>{{- "" -}}
     </span>{{- "" -}}
     <span class="{% if rtl %}pull-left{% else %}pull-right{% endif %}">{{- "" -}}
-        <a href="{{ url_for('help_page', pagename='about') }}">{{ _('about') }}</a>{{- "" -}}
+        <a href="{{ url_for('info', pagename='about', locale=current_locale) }}">{{ _('about') }}</a>{{- "" -}}
         <a href="{{ url_for('preferences') }}">{{ _('preferences') }}</a>{{- "" -}}
     </span>{{- "" -}}
 </div>