Convert reference book from a submodule to a subtree

[jyn514]

The reference is a submodule, it cannot actually be directly affected from here. This makes it annoying to update the documentation (which is already hard enough to get people to do). If we convert it to a subtree, we'd make it much easier to document language changes.

cc @ehuss since you're in charge of updating the books. I'm not sure who maintains the reference, I guess cc @rust-lang/lang-docs?

Originally posted by @workingjubilee in #85305 (comment)

[ehuss]

This was discussed a bit in rust-lang/reference#817. I guess I'm not 100% opposed, but it will mean more work for me, and more difficulty reviewing and maintaining it. I would lean towards not changing it unless the lang team wants to get more involved.

[jyn514]

I don't have a clear idea of the tradeoffs here so I'll defer to your experience :)