Install prettier and run on docs

[lmac-1]

#846 is adding some skills to do some automatic translations. Some of these skills run Prettier.

We need to run Prettier on the entire docs project and make sure PRs are easy to review. Make sure no regressions are added.

This is split into two PRs so each one is easy to review:

1. Install Prettier (branch prettier, based on Add docs maintenance agent: AGENTS.md, skills, and rule files #846): add prettier as a dev dependency, yarn format / yarn format:check scripts, a .prettierignore for generated and frozen files, and point the skills at yarn prettier --write <files>. No pages change.
2. Reformat everything (branch prettier-format-docs, based on PR 1): fix the few source issues Prettier trips on, then one formatting-only commit from yarn format, listed in .git-blame-ignore-revs so git blame skips it.

Could do
Formalise prettier and add a CI check (yarn format:check in test-deploy.yml), once PR 2 has landed.