Automate upstream schema sync with docs #177

Background:

Right now, updates here are still mostly manual, which means docs tend to drift from (Firefox) code. We have the opportunity to sync with upstream definitions, so to make things a little more stable, I'm going to make the following changes:

Tasks:

• feat: Check in schema & add sync script #178
• feat: Add weekly cron to fetch upstream schema #179
• feat: Add coverage script / automation #389

Motivation:

• At minimum, we should flag policies that exist in schema that do not have matching docs pages, and the inverse is also detectable, although rare.

• Longer term, the schema should drive generated reference elements on docs pages: policy description, defaults, compatibility, examples. The goal would be to have hand-written docs focus on explanation & context, gotchas etc. while the reference details come from upstream.