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/book.toml

@@ -6,13 +6,27 @@ src = "src"
 title = "Zed"
 site-url = "/docs/"
 
-[output.html]
+[build]
+extra-watch-dirs = ["../crates/docs_preprocessor"]
+
+# zed-html is a "custom" renderer that just wraps the
+# builtin mdbook html renderer, and applies post-processing
+# as post-processing is not possible with mdbook in the same way
+# pre-processing is
+# The config is passed directly to the html renderer, so all config
+# options that apply to html apply to zed-html
+[output.zed-html]
+command = "cargo run -p docs_preprocessor -- postprocess"
+# Set here instead of above as we only use it replace the `#description#` we set in the template
+# when no front-matter is provided value
+default-description = "Learn how to use and customize Zed, the fast, collaborative code editor. Official docs on features, configuration, AI tools, and workflows."
+default-title = "Zed Code Editor Documentation"
 no-section-label = true
 preferred-dark-theme = "dark"
 additional-css = ["theme/page-toc.css", "theme/plugins.css", "theme/highlight.css"]
 additional-js  = ["theme/page-toc.js", "theme/plugins.js"]
 
-[output.html.print]
+[output.zed-html.print]
 enable = false
 
 # Redirects for `/docs` pages.
@@ -24,7 +38,7 @@ enable = false
 # The destination URLs are interpreted relative to `https://zed.dev`.
 # - Redirects to other docs pages should end in `.html`
 # - You can link to pages on the Zed site by omitting the `/docs` in front of it.
-[output.html.redirect]
+[output.zed-html.redirect]
 # AI
 "/ai.html" = "/docs/ai/overview.html"
 "/assistant-panel.html" = "/docs/ai/agent-panel.html"