[docs] highlight source code linked from *automodule* content

sphinx.ext.viewcode won't highlight when 'highlight_language' [1] is set to
string 'none' [2]

[1] https://www.sphinx-doc.org/en/master/usage/extensions/viewcode.html
[2] https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-highlight_language



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