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
have people begun naming branches `ticket/NNNN` rather than `issue/NNNN`?