Lots of links in the tour use origin-relative links on the assumption
they will be used at "https://go.dev/", which works out badly if the
tour is being used offline. Links inside of "#appengine:" don't need to
be updated as they will be running inside the normal location.  Also
don't have to update links starting with "/tour" as they will work in
the offline and appengine modes.

I did consider updating the links to the place they currently redirect
for links like "https://go.dev/pkg/builtin" which ends up at
"https://pkg.go.dev/builtin" but it is redirecting with 307 Temporary
Redirect (unlike the 301 Moved Permanently "https://golang.org/" gets) so I
avoided those updates.

Update origin-relative links that will appear offline to be absolute
https://go.dev links.

Fixes golang/tour#1463