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
That's funny – it used to very much be practice to use `issue/NNNN`. Since ticket isn't necessarily an issue, maybe folks changed habits. All good, was just wondering because I never noticed the trend.