PA: Reflow glossary

This CL trivially converts our bulleted glossary into more heading-ful
markdown. This allows us to link directly to terms (anchored by their
headers).

Typographical minutiae:

*   bulleted lists are uniformly indented to four-space alignment.

*   single-bullet lists are converted to `promo` blocks.

Change-Id: I92dd814993c7f1ee1c0150726c9f0d142cbf7c5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5333952
Auto-Submit: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1270780}