[mod] doc - in sidebar, add links to the reST sources

Offer links to the reST sources (aka) html_show_sourcelink [1].  Links to the
reST source are sometimes very helpful, especially in our resT-Primer [2] :)

[1] https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_show_sourcelink
[2] https://searxng.github.io/searxng/dev/reST.html

Related-to: https://github.com/pallets/pallets-sphinx-themes/issues/32


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