Replace superseded docs for status codes

[jamesrobson-secondmind]

The links generated for status codes point to a doc that warns it is superseded. This replaces those links with the current standard.

Fixes #73

[sscherfke]

Why not switch to the MDN docs? They are a lot more readable and easier to understand.

This would also make all special cases except 449 obsolete.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Status

[big-r81]

Hey, one question, what's the blocker for merging this in?

[jamesrobson-secondmind]

Nothing on my side, it just needs a review.

[stevepiercy]

@jamesrobson-secondmind would you please:

• Update this branch against main
• Add a change log entry in doc/changelog.rst

Then I'll review and merge. It looks good. Thank you!

[jamesrobson-secondmind]

Done

[stevepiercy]

Thank you! I'll do the clean up from here.