doc: various documentation formatting fixes

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

Description of change

• 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 (https://github.com/addaleax/node/commit/edb5c6a71c1cb1b63b9a1014316d1ab80a91a24c)
• 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)