Exclude prompt files from format check and update instructions

The .tmpl.md files in prompts are garbled by mdformat due to embedded
html. This means the generated .md files shouldn't be formatted either,
or they'll no longer match.

- Update prompt file editing instructions to require manual duplicate edits of both .tmpl.md and .md files.
- Move .style.mdformat into subdirs, except for prompts/, to exclude the prompt files from formatting.

TAG=agy

CONV=836e7477-ec88-4c36-a0e4-f4ef14135a83

Change-Id: Ia2a6c83e66ea3eff62061a9a739e81dc05007a99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7871757
Commit-Queue: Joe Mason <joenotcharles@google.com>
Auto-Submit: Joe Mason <joenotcharles@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1635897}
NOKEYCHECK=True
GitOrigin-RevId: 12fec399c4928e7a497aa450ac4542e43f2963ee