refactor doc route url parsing

[brookslybrand]

After merging #190 I realized we had a bug with old tags

For example, if you add .md to the end of https://reactrouter.com/6.30.1/start/overview, the redirect will not work properly

The fix is corrected on staging, so https://reactrouterdotcomstaging.fly.dev/6.30.1/start/overview.md works

Additionally, I went ahead and added (ok ok, cursor added) some tests to ensure that we don't break the behavior in the future.

These tests made it easy to refactor the url parsing logic, which is still a bit hairy, but much more tame now.

@JuanJo4 if you have time I'd love a review since you implemented the original feature :)

[JuanJo4]

@brookslybrand ohhh that's a good catch! the tests look good and definetly helps to understand all of that url parsing logic, and that refactoring make it a lot easier to read through the code.

LGTM ✅