cmd/doc: add support for starting pkgsite instance for docs
This change adds a new flag "-http" to cmd/doc which enables starting
a pkgsite instance. -http will start a pkgsite instance and navigate to
the page for the requested package, at the anchor for the item
requested.
For #68106
Change-Id: Ic1c113795cb2e1035e99c89c8e972c799342385b
Reviewed-on: https://go-review.googlesource.com/c/go/+/628175
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sam Thanawalla <samthanawalla@google.com> Reviewed-by: Jonathan Amsterdam <jba@google.com> Reviewed-by: Alan Donovan <adonovan@google.com>