doc: installation.rst improve reST markup

- normalize section header markup
- limit all lines to a maximum of 79 characters
- add option -H to the sudo command used in code blocks
- place *WARNING* lines into a '.. warning::' admonition block
- make use of ``literal`` markup for monospace rendering
- drop useless indentation of lists

[1] https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html



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