refactor: Use axum for "mdbook serve"

[audunhalland]

This in my opinion brings mdbook's dependencies more in line with "the state of the art" of the Rust HTTP ecosystem.
It pulls in hyper v1 and uses tower-http for filesystem serving functions.

I've manually tested serve with the test_book:

• serving pages from the book works
• websocket-based auto-reload works
• 404 page works

I can't guarantee though that there are no regressions. I did not find integrations tests for the serve command.

[paolobarbolini]

FWIW this seems the right call, as warp is still on hyper v0.14 and looking at their repository it gets almost no maintenance.