Fix rustdoc warnings

[katrinafyi]

It would be good to fix these eventually. cargo doc --no-deps will report warnings.

It's a mix of unresolved links and non-escaped HTML. For example:

warning: unresolved link to `HostStats`
  --> lychee-lib/src/lib.rs:71:1
   |
13 | | //! }
   | |______^
...
71 | / /// Per-host rate limiting and concurrency control
   |
   = note: the link appears in this line:
           
           - [`HostStats`]: Statistics tracking for each host
              ^^^^^^^^^^^
   = note: no item named `HostStats` in scope
   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`

warning: unclosed HTML tag `stdin`
   --> lychee-lib/src/types/input/input.rs:215:58
    |
215 |     /// - Stdin input is returned as the special string "<stdin>"
    |                                                          ^^^^^^^

In particular, the invalid HTML leads to broken formatting like this: https://docs.rs/lychee-lib/0.23.0/lychee_lib/struct.RawUri.html#structfield.element

[thomas-zahner]

Thanks, I totally agree. As part of the issue we should also investigate why this isn't reported and caught in testing or CI. CI should fail if new warnings are emitted in the future.