## Summary
This PR reworks the `upstream_categories` mechanism that is only used
for documentation purposes to make it easier to generate docs using
`all_rules()`. The new implementation also relies on "tribal knowledge"
about rule codes, so it's not the best implementation, but gets us
forward.
Another option would be to change the rule-defining proc macros to allow
configuring an optional `RuleCategory`, but that seems more heavy-handed
and possibly unnecessary in the long run...
Draft since this builds on #5439.
cc @charliermarsh :)
24bcbb85a1