Smarter docs URL handling

### What needs to get done

If the docs URL is a slug or something, match it to the application's docs and put that in.

This will probably initially need to be a spike

### Why it needs to get done

To let libraries raise exceptions without the application having to catch and reraise them with a docs URL

(Suggested by @sergiusens )

No files, tests, or entry points are named. Start by tracing how exceptions and documentation URLs are currently handled, then clarify how a slug should map to the application's docs and what the spike must establish. Done means an agreed approach for allowing libraries to provide docs URLs without application-level catch-and-reraise handling.

Written by the indexing model from the issue text.