doc: Enable Markdown doc compilation

[wenzeslaus]

The initial commit simply uncoments the Markdown compilation lines from #3849. This requires the actual Markdown files to be present, so this breaks the CI. This will stay like this for now. The PR is mean to be tested together with the Markdown files generated by script in #4620 (switch to this branch after generating the files).

Subsequent commits will add fixes of the compilation (missing tools, files, etc.).

To test this (assuming you already have the Markdown files):

Get virtual environment in some way (venv or pipenv).

Install dependencies:

pip install -r man/mkdocs/requirements.txt

Compile everything:

make distclean && ./configure ... && make
(cd man && make build-mkdocs)

[wenzeslaus]

This now builds Markdown files in dist based on the HTML-based .md files in the code (without a need to generate the files). make build-mkdocs (see above) can run as well.

[wenzeslaus]

When this is merged, ./utils/grass_html2md.sh can be easily tested in action with or without changes in #5054.