Link to the contributing guide if newsfragment check fails

[anoadragon453]

There are now multiple cases of community contributors being confused about what to do when the newsfragment check fails:

• Add a clarification for the public_baseurl property. #7906 (comment)
• Allow guests to operate in encrypted rooms #7314 (comment)

When python -m towncrier.check --compare-with=origin/develop fails, it exits with error code 1, otherwise exit code 0 when the check succeeds. We use this to echo a link to the changelog contributing guide when the check fails, to hopefully allow contributors to be informed and solve the issue themselves.

[anoadragon453]

I've realised this would make more sense to sit in the pipeline instead: matrix-org/pipelines#82