doc: various documentation formatting fixes

* Fix markdown code sample in releases.md, it was <a id="x.y.x></a>"
* Fix some markdown errors, e.g. in changelogs
* Fix broken defs links, e.g. in domain-postmortem.md
* Fix other broken refs, by addaleax
* Add links to some defs that were present but not linked to
* Remove dead defs
* Move defs to the bottom (one file affected)
* Add language indicators to all code blocks, using `txt` when no
specific language could be chosen
* Some minor formatting changes (spaces, ident, headings)

PR-URL: https://github.com/nodejs/node/pull/7637


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Roman Reiss <me@silverwind.io>