doc: improve generated scion and scion-pki documentation

[oncilla]

Unify the documentation generation command for scion and scion-pki. Instead of rst, produce markdown which is easier to process.

Properly handled references between the individual commands. This way, we even get proper indexing in the toc tree on the side.

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This change is 

[matzf]

Can we defer this until #4325?
If we add a customized gendoc tool, I'd prefer it to spit out something similar to manually written RST using all the nice sphinx features for layout and cross-referencing of command line options (e.g. .. option::).

[matzf]

I just came across sphinx-contrib/datatemplates. This allows to write a template .rst file and render it using data from yaml/json files.
Perhaps this could be a way to allow generating the content of commandline docs as we currently do, but allow keeping the markup styling as part of the documentation tree.