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