tour: relative links don't work outside of /tour for offline tour

[crisman]

Context: https://go.dev/tour/moretypes/15

Lots of links in the tour using 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. For example we have the append documentation going to

http://127.0.0.1:3999/pkg/builtin/#append

rather than

https://go.dev/pkg/builtin/#append

Lots of pkg/ links, also /blog, /doc, /talks, /cmd, & / directly.