[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/info/en/search-syntax.md

@@ -0,0 +1,35 @@
+# Search syntax
+
+SearXNG allows you to modify the default categories, engines and search language
+via the search query.
+
+Prefix `!` to set category and engine names.
+
+Prefix: `:` to set the language.
+
+Abbrevations of the engines and languages are also accepted.  Engine/category
+modifiers are chainable and inclusive.  E.g. with {{search('!map !ddg !wp paris')}}
+search in map category **and** duckduckgo **and** wikipedia for
+`paris`.
+
+See the {{link('preferences', 'preferences')}} for the list of engines,
+categories and languages.
+
+## Examples
+
+Search in wikipedia for `paris`:
+
+* {{search('!wp paris')}}
+* {{search('!wikipedia paris')}}
+
+Search in category `map` for `paris`:
+
+* {{search('!map paris')}}
+
+Image search:
+
+* {{search('!images Wau Holland')}}
+
+Custom language in wikipedia:
+
+* {{search(':fr !wp Wau Holland')}}