Make all prdoc valid and add CI job to check prdoc with `prdoc check` (#7543)

Some prdoc are invalid, `prdoc check` is failing for them. This also
broke usage of parity-publish.

This PR fixes the invalid prdoc, and add a ci job to check the prdoc are
valid. I don't think the check is unstable considering it is a simple
yaml check, so I put the job as required.

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Co-authored-by: Alexander Samusev <41779041+alvicsam@users.noreply.github.com>