cmd/doc: use golang.org/x/pkgsite/cmd/internal/doc to start server
This change switches the pkgsite command invoked to start a pkgsite
server from golang.org/x/pkgsite/cmd/pkgsite to
golang.org/x/pkgsite/cmd/internal/doc. The doc command is a simplified
version of cmd/pkgsite that changes some options to improve the user
experience. For example, it limits logging informational log messages,
doesn't always expect to find modules (for example if we're outside of a
module getting documentation for the standard library), and it takes the
address of the page to open in the browser (which simplifies waiting for
the server to start listening).
Fixes #68106
Change-Id: I667a49d03823242fa1aff333ecb1c0f198e92412
Reviewed-on: https://go-review.googlesource.com/c/go/+/674158
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Jonathan Amsterdam <jba@google.com> Reviewed-by: David Chase <drchase@google.com>