Process Follow-ups

[gkatsev]

Follow-ups:

• [ ] Add "Implemented" folder or Stages like TC39 proposals and update the process to include these
  • [ ] update proposal to include a link to the implementation PR
• [ ] Add https://github.com/muxinc/custom-video-element as the canonical implementation of these proposals, but not the only implementation
• [ ] Update the wording in the README to be more general of "Media Elements"
• [ ] Clarify how to call a CfC (comment on the PR)

Originally posted by @gkatsev in https://github.com/video-dev/media-ui-extensions/issues/5#issuecomment-1424917735