Fix broken rustdoc links across ockam crates

[thomcc]

If you cd into some of our crates and run cargo doc, you'll get warnings (or errors) that the documentation uses links that are no longer valid. For example

$ cd implementations/rust/ockam/ockam
$ cargo doc

... <lots of build output> ...

warning: unresolved link to `ockem_core::RouteBuilder`
   --> /Users/thom/src/ockam/implementations/rust/ockam/ockam_node/src/context.rs:255:27
    |
255 |     /// [`RouteBuilder`]: ockem_core::RouteBuilder
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^ no item named `ockem_core` in scope

...

error: unresolved link to `UserParser`
  --> src/protocols/parser.rs:16:20
   |
16 | /// [`UserParser`](UserParser) instead!**
   |                    ^^^^^^^^^^ no item named `UserParser` in scope
   |

Pretty much all of these are fair game. If it isn't clear what the documentation should reference instead, it's acceptable to make it no longer a link, as after this I'll be filing another issue for fixing stale references like Foo in docs (with more of a guide for how to figure that out).

[SanjoDeundiak]

@metaclips could you please investigate how to check those in CI?