Clean mkdocs

[etiennebacher]

Close #113 Close #114 Close #115

@vincentarelbundock can you check if python -m mkdocs works on your system?

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[vincentarelbundock]

Can't test now, but on at least one of my machines I have the call python3

[etiennebacher]

That's annoying, I don't know how to check that mkdocs is installed on all OS

[vincentarelbundock]

Maybe we do our best to check, then try variations:

mkdocs run .... python mkdocs build... python3 mkdocs build...

then we catch errors and say something like "Did you install...?" and print the actual error message that we caught.

[etiennebacher]

Looks like python3 and python are just here because sometimes there's also a python v2 installation: https://superuser.com/questions/1440218/whats-the-difference-between-the-keywords-python-and-python3-in-command-lin

Using python3 -m everywhere should work.

The mkdocs workflow still doesn't work because of #116 though

[vincentarelbundock]

see my proposal in that other issue for a possible workflow.

[etiennebacher]

This PR doesn't actually change the workflow, just makes mkdocs errors pop up earlier in the process if mkdocs is not here, so I'll merge it now