Merge branch 'ab/doc-replace-gmane-links' into maint

The Web interface to gmane news archive is long gone, even though
the articles are still accessible via NTTP.  Replace the links with
ones to public-inbox.org.  Because their message identification is
based on the actual message-id, it is likely that it will be easier
to migrate away from it if/when necessary.

* ab/doc-replace-gmane-links:
  doc: replace more gmane links
  doc: replace a couple of broken gmane links