This adds markdown link checking in th documentation sources
to the daily-nightly CI jobs.
The purpose of this is to catch breaking external links to help
keep out documentation clean.
Catching breaking external links is best done in the daily CI runs
instead of checking links in PRs. link checking in PRs would prevent
changes in PRs from newly introducing broken links.
Signed-off-by: Michael Adam <obnox@samba.org>