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>