Update anchor links and all internal links

GitHub changed how they do header links, so:

- make all our anchor links kebab-case
- update all other internal document links
- remove the ones that are in headings
- add a lookup/override to fix things that either were inflected
  differently or we were using an anchor link that didn't match the
  title

Work done by a special script (attached to the pull request and with
credit to @handrews), and checked with markdownlint.

Author: lornajane