Closes: https://github.com/searxng/searxng/issues/4370
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

We have built up detailed documentation of the *settings* and the *engines* over
the past few years.  However, this documentation was still spread over various
chapters and was difficult to navigate in its entirety.

This patch rearranges the Settings & Engines documentation for better
readability.

To review new ordered docs::

   make docs.clean docs.live

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

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>

settings.yml snippets have no place in the user docs.

The ? search operator has been broken for some time and
currently only raises the question why it's still there.

## Context ##

The query "Paris !images" searches for "Paris" in the "images" category.

Once upon a time Searx supported "Paris ?images" to search for "Paris"
in the currently enabled categories and the "images" category.

The feature makes sense ... the ? syntax does not.
We will hopefully introduce a +!images syntax in the future.

Fixes #702.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>