Add meta description tag to docs pages (#35112)

Closes #ISSUE

Adds basic frontmatter support to `.md` files in docs. The only
supported keys currently are `description` which becomes a `<meta
name="description" contents="...">` tag, and `title` which becomes a
normal `title` tag, with the title contents prefixed with the subject of
the file.

An example of the syntax can be found in `git.md`, as well as below

```md
---
title: Some more detailed title for this page
description: A page-specific description
---

# Editor
```

The above will be transformed into (with non-relevant tags removed)

```html
<head>
    <title>Editor | Some more detailed title for this page</title>
    <meta name="description" contents="A page-specific description">
</head>
<body>
<h1>Editor</h1>
</body>
```

If no front-matter is provided, or If one or both keys aren't provided,
the title and description will be set based on the `default-title` and
`default-description` keys in `book.toml` respectively.

## Implementation details

Unfortunately, `mdbook` does not support post-processing like it does
pre-processing, and only supports defining one description to put in the
meta tag per book rather than per file. So in order to apply
post-processing (necessary to modify the html head tags) the global book
description is set to a marker value `#description#` and the html
renderer is replaced with a sub-command of `docs_preprocessor` that
wraps the builtin `html` renderer and applies post-processing to the
`html` files, replacing the marker value and the `<title>(.*)</title>`
with the contents of the front-matter if there is one.

## Known limitations

The front-matter parsing is extremely simple, which avoids needing to
take on an additional dependency, or implement full yaml parsing.

* Double quotes and multi-line values are not supported, i.e. Keys and
values must be entirely on the same line, with no double quotes around
the value.

The following will not work:

```md
---
title: Some
 Multi-line
 Title
---
```

* The front-matter must be at the top of the file, with only white-space
preceding it

* The contents of the title and description will not be html-escaped.
They should be simple ascii text with no unicode or emoji characters

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Katie Greer <katie@zed.dev>

docs/README.md

@@ -69,3 +69,64 @@ Templates are just functions that modify the source of the docs pages (usually w
 - Template Trait: crates/docs_preprocessor/src/templates.rs
 - Example template: crates/docs_preprocessor/src/templates/keybinding.rs
 - Client-side plugins: docs/theme/plugins.js
+
+## Postprocessor
+
+A postprocessor is implemented as a sub-command of `docs_preprocessor` that wraps the builtin `html` renderer and applies post-processing to the `html` files, to add support for page-specific title and meta description values.
+
+An example of the syntax can be found in `git.md`, as well as below
+
+```md
+---
+title: Some more detailed title for this page
+description: A page-specific description
+---
+
+# Editor
+```
+
+The above will be transformed into (with non-relevant tags removed)
+
+```html
+<head>
+  <title>Editor | Some more detailed title for this page</title>
+  <meta name="description" contents="A page-specific description" />
+</head>
+<body>
+  <h1>Editor</h1>
+</body>
+```
+
+If no front-matter is provided, or If one or both keys aren't provided, the title and description will be set based on the `default-title` and `default-description` keys in `book.toml` respectively.
+
+### Implementation details
+
+Unfortunately, `mdbook` does not support post-processing like it does pre-processing, and only supports defining one description to put in the meta tag per book rather than per file. So in order to apply post-processing (necessary to modify the html head tags) the global book description is set to a marker value `#description#` and the html renderer is replaced with a sub-command of `docs_preprocessor` that wraps the builtin `html` renderer and applies post-processing to the `html` files, replacing the marker value and the `<title>(.*)</title>` with the contents of the front-matter if there is one.
+
+### Known limitations
+
+The front-matter parsing is extremely simple, which avoids needing to take on an additional dependency, or implement full yaml parsing.
+
+- Double quotes and multi-line values are not supported, i.e. Keys and values must be entirely on the same line, with no double quotes around the value.
+
+The following will not work:
+
+```md
+---
+title: Some
+  Multi-line
+  Title
+---
+```
+
+And neither will:
+
+```md
+---
+title: "Some title"
+---
+```
+
+- The front-matter must be at the top of the file, with only white-space preceding it
+
+- The contents of the title and description will not be html-escaped. They should be simple ascii text with no unicode or emoji characters