[mod] replace Markdown parser mistletoe by markdown-it-py

There are several reasons why we should prefer markdown-it-py over mistletoe:

- Get identical rendering results in SearXNG's `/info` pages and the SearXNG's
  project documentation which is build by Sphinx-doc.

  In the Sphinx-doc we use the MyST parser to render Markdown and the MyST
  parser itself is built on top of the markdown-it-py package.

- markdown-it-py has a typographer that supports *replacements*
  and *smartquotes* (e.g. em-dash, copyright, ellipsis, ...) [1]

- markdown-it-py is much more flexible compared to mistletoe [2]

- markdown-it-py is the fastest CommonMark compliant parser in python [3]

[1] https://markdown-it-py.readthedocs.io/en/latest/using.html#typographic-components
[2] https://markdown-it-py.readthedocs.io/en/latest/plugins.html
[3] https://markdown-it-py.readthedocs.io/en/latest/other.html#performance



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