Handle :orphan: tags

[nikophil]

I think :orphan: tag is not handle by the parser https://www.sphinx-doc.org/en/master/usage/restructuredtext/field-lists.html#metadata

After the build process is complete, we're performing some checks to see if every rst file has its equivalent html file. If some file marked as :orphan: doesn't have a matching html file, then we don't need to send a warning.

I don't really know if this check process should be in the core parser or in our side, but if we say it's in our land, that would be nice to have this information in meta...

what do you think ?