check for broken links, etc., in documentation in CI
Created by: sqs
Runs docsite check
to find broken links in the doc/
Markdown files in CI. Running docsite check
at the top level also runs the checker.
If there is a broken link, CI will fail with an error like:
index.md: broken link to /user/index2
1 problems found
Helps with #1515 (but does not fix all broken links identified there).