[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/.gitignore

@@ -0,0 +1 @@
+*.md

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

docs/user/search_syntax.rst

@@ -1,39 +0,0 @@
-
-.. _search-syntax:
-
-=============
-Search syntax
-=============
-
-SearXNG allows you to modify the default categories, engines and search language
-via the search query.
-
-Prefix ``!``
-  to set Category/engine
-
-Prefix: ``:``
-  to set language
-
-Abbrevations of the engines and languages are also accepted.  Engine/category
-modifiers are chainable and inclusive (e.g. with :search:`!it !ddg !wp qwer
-<?q=%21it%20%21ddg%20%21wp%20qwer>` search in IT category **and** duckduckgo
-**and** wikipedia for ``qwer``).
-
-See the :search:`/preferences page <preferences>` for the list of engines,
-categories and languages.
-
-Examples
-========
-
-Search in wikipedia for ``qwer``:
-
-- :search:`!wp qwer <?q=%21wp%20qwer>` or
-- :search:`!wikipedia qwer :search:<?q=%21wikipedia%20qwer>`
-
-Image search:
-
-- :search:`!images Cthulhu <?q=%21images%20Cthulhu>`
-
-Custom language in wikipedia:
-
-- :search:`:hu !wp hackerspace <?q=%3Ahu%20%21wp%20hackerspace>`