Move wpt README to firefox-source-docs

Add gecko-specific wpt docs to firefox-source-docs. This is instead of
the prior setup where we put the generic upstream docs on
firefox-source-docs and the gecko-specific documentation isn't
presented at all. It also avoids the problem where the upstream docs
don't build in the Firefox setup due to configuration differences.

Differential Revision: https://phabricator.services.mozilla.com/D133898

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1657366
gecko-commit: 630013ad7f37ccb206559c998a5714b812f22636
gecko-reviewers: karlcow