Rollup merge of #124148 - notriddle:notriddle/reference, r=GuillaumeGomez

rustdoc-search: search for references

This feature extends rustdoc with syntax and search index information for searching borrow references. Part of rust-lang/rust#60485

## Preview

- [`&mut`](https://notriddle.com/rustdoc-html-demo-11/reference/std/index.html?search=%26mut)
- [`&Option<T> -> Option<&T>`](https://notriddle.com/rustdoc-html-demo-11/reference/std/index.html?search=%26Option%3CT%3E%20-%3E%20Option%3C%26T%3E)
- [`&mut Option<T> -> Option<&mut T>`](https://notriddle.com/rustdoc-html-demo-11/reference/std/index.html?search=%26mut%20Option%3CT%3E%20-%3E%20Option%3C%26mut%20T%3E)

Updated chapter of the book: https://notriddle.com/rustdoc-html-demo-11/reference/rustdoc/read-documentation/search.html

## Motivation

See rust-lang/rust#119676

## Guide-level explanation

You can't search by lifetimes, but other than that it's the same syntax references normally use.

## Reference-level description

<table>
<thead>
  <tr>
    <th>Shorthand</th>
    <th>Explicit names</th>
  </tr>
</thead>
<tbody>
  <tr><td colspan="2">Before this PR</td></tr>
  <tr>
    <td><code>[]</code></td>
    <td><code>primitive:slice</code> and/or <code>primitive:array</code></td>
  </tr>
  <tr>
    <td><code>[T]</code></td>
    <td><code>primitive:slice&lt;T&gt;</code> and/or <code>primitive:array&lt;T&gt;</code></td>
  </tr>
  <tr>
    <td><code>!</code></td>
    <td><code>primitive:never</code></td>
  </tr>
  <tr>
    <td><code>()</code></td>
    <td><code>primitive:unit</code> and/or <code>primitive:tuple</code></td>
  </tr>
  <tr>
    <td><code>(T)</code></td>
    <td><code>T</code></td>
  </tr>
  <tr>
    <td><code>(T,)</code></td>
    <td><code>primitive:tuple&lt;T&gt;</code></td>
  </tr>
  <tr>
    <td><code>(T, U -> V, W)</code></td>
    <td><code>fn(T, U) -> (V, W)</code>, Fn, FnMut, and FnOnce</td>
  </tr>
  <tr><td colspan="2">New additions with this PR</td></tr>
  <tr>
    <td><code>&</code></td>
    <td><code>primitive:reference</td>
  </tr>
  <tr>
    <td><code>&mut</code></td>
    <td><code>primitive:reference&lt;keyword:mut&gt;</td>
  </tr>
  <tr>
    <td><code>&T</code></td>
    <td><code>primitive:reference&lt;T&gt;</td>
  </tr>
  <tr>
    <td><code>&mut T</code></td>
    <td><code>primitive:reference&lt;keyword:mut, T&gt;</td>
  </tr>
</tbody>
</table>

### Search query grammar

<code><pre><strong>borrow-ref = AMP *WS [MUT] *WS [arg]</strong>
arg = [type-filter *WS COLON *WS] (path [generics] / slice-like / tuple-like / <strong>borrow-ref</strong>)</pre></code>

```
AMP = "&"
MUT = "mut"
```

## Future direction

As described in rust-lang/rust#118194 and rust-lang/rust#119676

* The remaining type expression grammar (this is another step in the type expression grammar: `ReferenceType` is now supported)
* Search subtyping and traits

Author: GuillaumeGomez