"Refactor" how we maintain documentation on PPxxxx diagnostics codes

We currently issue new PPxxxx numbers ad hoc, and maintain wiki pages for them. This is problematic for a number of reasons:

• Wiki pages cannot be edited by the general public, but only Prepack team members
• There's a race when issuing new PPxxxx numbers, as the PRs are disconnected from editing wiki pages
• As a result, discrepancies have crept in in the past, see e.g. #2439

Proposal:

• Migrate all PPxxxx wiki pages to files in a new directory website/diagnostics/en-US
• Have a new file website/diagnostics.html which links to all the individual diagnostics issues
• Consider generating this file with some script that can be run from the scripts/publish-gh-pages.sh script
• Use some on-the-fly markdown processor in JavaScript to render the individual files in the website/diagnostics directory
• (Nice to have) write some kind of linter that ensures that every mention of PPxxxx has an associated file in the website/diagnostics directory

Advantages:

• Races/conflicts between allocating the same new PPxxxx number will be detected before a PR can land
• Diagnostics help text will get reviewed as part of a PR
• Non-prepack team members can contribute diagnostics help
• It will be way easier to ensure that PPxxxx actually have documentation (ideally in an automated fashion with some kind of linter)