Error or warn on links that start with `#` and recommend `self#` or `Self#` instead

Rustdoc should error or warn on links that start `#` and recommend `self#` for a module or `Self#` for a type.

_Originally posted [here](https://github.com/rust-lang/rust/pull/76497#discussion_r485908548) and [here](https://github.com/rust-lang/rust/pull/76497#discussion_r485968957)._

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Cc @jyn514

@rustbot modify labels: A-intra-doc-links T-rustdoc