chore(developer): validate URLs found in compiler messages against /developer/docs/help paths

Looking at what we already have with `kmc message`, I think we should go with `https://kmn.sh/km#####` as our redirect (already baked in). It's also a redirect with intent for URLs to be permanent so if help site layout changes then it will just mean updating the redirect.

I am not so sure about URLs embedded in message details. Am considering leaving those as-is, and declaring them to be an exception to the rule, because otherwise we have to add a lot of redirects with little benefit.

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/pull/13156#discussion_r1945986888_

As we have URLs hardcoded into the compiler messages that reference pages on help.keyman.com, in general those will be pointing to /developer/ pages, many of which should be accessible in the /docs/help folder. So a test to verify against those paths should be very achievable.
* Stretch goal: check also product pages because those are all accessible as well.
* Additional stretch: Report on other URLs found in the help to ease manual verification