Move to a single, canonical URL pattern for AEPs

[earth2marsh]

Today the site generator supports both the numbered resource (example: https://aep.dev/135 ) and the slug resource (example: https://aep.dev/delete/ ).

As discussed in last week's call, continuing down this path this will split traffic and complicate both search results and web metrics. Going forward, each AEP should have one canonical URL based on the (immutable) AEP number.

This has a few implications (happy to split out as sub-tasks if necessary!):

• Every AEP page should have only one canonical URL
• Links in AEP document content should prefer the numbered pattern
• An AEP may be aliased to one (or possibly more) slugs, which will each re-direct to the canonical numbered URL pattern
• The rule numbers should be included in the sidenav list of AEPs
• Consider whether slugs should appear in the keywords meta field (not sure how useful those are any more)
• Any AEP number reference that begins with 0 (example: AEP-0164), should probably have any leading zero(s) stripped.