doc(dev): reST/sphinx add tabbed views extension (sphinx_tabs.tabs)

See issue #1785:

  idea: in the doc, provide installation instructions with one tab per
  distrubution

preview (don't bookmark):

  https://return42.github.io/searx/dev/reST.html#tabbed-views



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