[URL Protocol Handler Registration] The explainer for protocol handlers should graduate from this repo.

[fabiorocha]

The protocol handler explainer has been stable for a couple of months now and the work has now been spec'd (currently living in the WICG manifest-incubation repo for lack of multiple independent implementations) and implemented in Chromium. It seems like it should be moved to the Alumni section on this repo, so that possible future discussions are tracked on the incubation repo only.

[captainbrosset]

@diekus just making sure this shows up on your radar. Will you be the one doing this?