Update hacker guide: add more info, fix broken links.

This pretty much ports things from https://github.com/scala/scala/pull/3830 to relevant sections of the hacker guide. It also fixes some broken links.

/cc @xeno-by @heathermiller @retronym @adriaanm @som-snytt @paulp
Tbh, I've never seen a branch named `issue/NNNN`. I've seen a lot of `ticket/NNNN` though. Here's some branch listings from the contributors: https://github.com/retronym/scala/branches, https://github.com/adriaanm/scala/branches, https://github.com/xeno-by/scala/branches
In fact, two paragraphs down from here @xeno-by introduces a branch `ticket/6725`.