Add note about pre-commit mirror versions to docs/precommit.md

[kaechele]

Description

Add notes on how to use prettier with the pre-commit mirror for when certain stable version tags are unavailable.

Checklist

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory).
• (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[louwers]

pre-commit no longer supports prettier, so this section can be removed entirely

#16229

[fisker]

I agree that we should just remove it if Prettier doesn't work with precommit anymore.