detect absolute links

[DiegoVinasco]

There is a flaw with remark validate links...

Bascially, Remark doesn't validate absolute links (which are accepted by github in markdown documents). It only checks relative links or URLs.

This means that we might still have some broken links. This was worked around in the private repository here by using a sed command with some pipes to prefix absolute markdown links with a given URL:

export BASE_URI='https://github.com/VCityTeam/VCity/blob/master'
files=$(find . -name '*.md')
for f in $files
do
  grep -E '\[{1}.*\]{1}\({1}/{1}.*\){1}' "$f" | sed -i "s|](/|]($BASE_URI/|g" "$f"
done
remark -u validate-links .

I think it would be a good idea to do the same in UD-Viz. Maybe in a different PR? Any thoughts?

Originally posted by @DiegoVinasco in #548 (comment)