Fixup plugin internal doc links & element yaml inlines

When moving over the pip plugins https://gitlab.com/BuildStream/bst-plugins-experimental/-/merge_requests/78 it was noted that most of the plugins that have been previously transitioned have broken links to doc pages (as they're no longer in the same codebase), along with literalinclude paths to associated .yaml for element plugins.

For the links I think it makes sense to point at the equivalent documentation on https://docs.buildstream.build/master as I did for the pip plugins.