Rollup merge of #120443 - GuillaumeGomez:footnote-def-improvement, r=fmease

Fixes footnote handling in rustdoc

Fixes #100638.

You can now declare footnotes like this:

```rust
//! Reference to footnotes A[^1], B[^2] and C[^3].
//!
//! [^1]: Footnote A.
//! [^2]: Footnote B.
//! [^3]: Footnote C.
```

r? `@notriddle`

Author: GuillaumeGomez