x/tools/cmd/godoc: add support for hotlinks

[dsnet]

Per #18342 (comment), the addition of hotlinks was accepted to be added to godoc. Filing this as a separate issue since the previous issue is conflating sections and hotlinks.

The design for it is in my comment here: #18342 (comment)

[kegsay]

Is there any update on this? I seem to recall you did this already and even had it hosted somewhere.

[dmitshur]

@dsnet has created a prototype for this, and this proposal is accepted, so it will be integrated into x/tools/cmd/godoc code. It hasn't happened yet, but updates will be posted here when there's progress.