go/doc, x/pkgsite: add comparable as predeclared type

[fzipp]

CL 359778 added any as predeclared type to go/doc, but not comparable.

This is the reason why pkg.go.dev (via its internal copy of go/doc) doesn't linkify comparable, e.g. here.

/cc @findleyr

[findleyr]

Indeed, thank you for the investigation! Since this is an easy fix, I think we can probably get this into go/doc for 1.18.

CC @griesemer @jba

[gopherbot]

Change https://go.dev/cl/385114 mentions this issue: go/doc: mark comparable predeclared

[findleyr]

Reopening to track the equivalent change for pkgsite.

[gopherbot]

Change https://go.dev/cl/385198 mentions this issue: internal/godoc/internal/doc: mark comparable as predeclared