docs: An attempt to fix broken links.

[saona-raimundo]

Tries to fix #257 (fixes all but one).

There are still many links that I could not fix in the fluent crate, but they are not a problem in the fluent-bundle crate. I do not understand why these problems arise.

Relevant readings on Rust docs:

• Last addition to rustdoc capacities: https://rust-lang.github.io/rfcs/1946-intra-rustdoc-links.html
• Frontend/backend crates pattern: https://users.rust-lang.org/t/cross-crate-documentation-links-in-a-workspace/67588
• Help linking to an external crate (for which we want to link only on the documentation level): https://github.com/rust-lang/api-guidelines/discussions/186
• Open issue on how to do that: https://github.com/rust-lang/rust/issues/74481

Useful tools

I can recommend trying cargo-deadlinks. It helped find broken links, even if the error message is not the best.