Fix Broken Doc links and Deprecate FromUtf8Error in oco.rs

[zoomiti]

Potentially fixes #859

The relative paths for some doc links comes inspired from what tokio does for its workspace links. See here for an example.

[benwis]

Can you run cargo fmt on your PR?

[benwis]

Thanks for looking into this!

[gbj]

I'm removing the deprecation with #2362. Adding deprecation warnings can be tricky, since adding them in a patch release can cause previously-passing CI to fail if it is set up to deny based on clippy warnings. This was happening in our own repo after this change; I would not have expected it to be a problem, but clippy was warning about the use of it in leptos_reactive itself when run in leptos_dom etc. You can see the warning showing up in CI for recent PRs.

warning: use of deprecated enum `oco::FromUtf8Error`: This error was intended for a `TryFrom` implementation for `Oco`. `Oco` will likely move to its own crate after 0.7 so this type will be removed in the future
   --> leptos_reactive/src/oco.rs:450:10
    |
450 | pub enum FromUtf8Error {
    |          ^^^^^^^^^^^^^