Use documented crate (or custom proc macro) to generate documentation for editor config options

[nik-rev]

TLDR: I propose automatically generating the book/src/editor.md page by moving the documentation from there into helix-view/src/ui/editor.rs doc comments.

Then generating markdown from these doc comments.

---

Helix currently has documentation for the some of same things in 2 different places:

• in the source code as doc comments
• in markdown, in the book

Some of the documentation is automatically generated, but not all of it. For example, consider this FilePickerConfig:

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "kebab-case", default, deny_unknown_fields)]
pub struct FilePickerConfig {
    /// IgnoreOptions
    /// Enables ignoring hidden files.
    /// Whether to hide hidden files in file picker and global search results. Defaults to true.
    pub hidden: bool,
    /// Enables following symlinks.
    /// Whether to follow symbolic links in file picker and file or directory completions. Defaults to true.
    pub follow_symlinks: bool,
    /// Hides symlinks that point into the current directory. Defaults to true.
    pub deduplicate_links: bool,
    /// Enables reading ignore files from parent directories. Defaults to true.
    pub parents: bool,
    /// Enables reading `.ignore` files.
    /// Whether to hide files listed in .ignore in file picker and global search results. Defaults to true.
    pub ignore: bool,
    /// Enables reading `.gitignore` files.
    /// Whether to hide files listed in .gitignore in file picker and global search results. Defaults to true.
    pub git_ignore: bool,
    /// Enables reading global .gitignore, whose path is specified in git's config: `core.excludefile` option.
    /// Whether to hide files listed in global .gitignore in file picker and global search results. Defaults to true.
    pub git_global: bool,
    /// Enables reading `.git/info/exclude` files.
    /// Whether to hide files listed in .git/info/exclude in file picker and global search results. Defaults to true.
    pub git_exclude: bool,
    /// WalkBuilder options
    /// Maximum Depth to recurse directories in file picker and global search. Defaults to `None`.
    pub max_depth: Option<usize>,
}

Which has the following markdown documentation:

### `[editor.file-picker]` Section

Set options for file picker and global search. Ignoring a file means it is
not visible in the Helix file picker and global search.

All git related options are only enabled in a git repository.

| Key | Description | Default |
|--|--|---------|
|`hidden` | Enables ignoring hidden files | `true`
|`follow-symlinks` | Follow symlinks instead of ignoring them | `true`
|`deduplicate-links` | Ignore symlinks that point at files already shown in the picker | `true`
|`parents` | Enables reading ignore files from parent directories | `true`
|`ignore` | Enables reading `.ignore` files | `true`
|`git-ignore` | Enables reading `.gitignore` files | `true`
|`git-global` | Enables reading global `.gitignore`, whose path is specified in git's config: `core.excludesfile` option | `true`
|`git-exclude` | Enables reading `.git/info/exclude` files | `true`
|`max-depth` | Set with an integer value for maximum depth to recurse | Unset by default

Ignore files can be placed locally as `.ignore` or put in your home directory as `~/.ignore`. They support the usual ignore and negative ignore (unignore) rules used in `.gitignore` files.

Additionally, you can use Helix-specific ignore files by creating a local `.helix/ignore` file in the current workspace or a global `ignore` file located in your Helix config directory:
- Linux and Mac: `~/.config/helix/ignore`
- Windows: `%AppData%\helix\ignore`

Example:

```ini
# unignore in file picker and global search
!.github/
!.gitignore
!.gitattributes
```

We could generate that markdown documentation using a crate like documented. To do that, merge the markdown documentation with the source code doc comments:

/// Set options for file picker and global search. Ignoring a file means it is
/// not visible in the Helix file picker and global search.
///
/// All git related options are only enabled in a git repository.
///
/// {{#include ./generated/file-picker-config-options.md}}
///
/// Ignore files can be placed locally as `.ignore` or put in your home directory as `~/.ignore`. They support the usual ignore and negative ignore (unignore) rules used in `.gitignore` files.
///
/// Additionally, you can use Helix-specific ignore files by creating a local `.helix/ignore` file in the current workspace or a global `ignore` file located in your Helix config directory:
/// - Linux and Mac: `~/.config/helix/ignore`
/// - Windows: `%AppData%\helix\ignore`
///
/// Example:
///
/// ```ini
/// # unignore in file picker and global search
/// !.github/
/// !.gitignore
/// !.gitattributes
/// ```
#[cfg_attr(doc_gen, derive(Documented, DocumentedFields))]
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "kebab-case", default, deny_unknown_fields)]
pub struct FilePickerConfig {
    /// IgnoreOptions
    /// Enables ignoring hidden files.
    /// Whether to hide hidden files in file picker and global search results. Defaults to true.
    pub hidden: bool,
    /// Enables following symlinks.
    /// Whether to follow symbolic links in file picker and file or directory completions. Defaults to true.
    pub follow_symlinks: bool,
    /// Hides symlinks that point into the current directory. Defaults to true.
    pub deduplicate_links: bool,
    /// Enables reading ignore files from parent directories. Defaults to true.
    pub parents: bool,
    /// Enables reading `.ignore` files.
    /// Whether to hide files listed in .ignore in file picker and global search results. Defaults to true.
    pub ignore: bool,
    /// Enables reading `.gitignore` files.
    /// Whether to hide files listed in .gitignore in file picker and global search results. Defaults to true.
    pub git_ignore: bool,
    /// Enables reading global .gitignore, whose path is specified in git's config: `core.excludefile` option.
    /// Whether to hide files listed in global .gitignore in file picker and global search results. Defaults to true.
    pub git_global: bool,
    /// Enables reading `.git/info/exclude` files.
    /// Whether to hide files listed in .git/info/exclude in file picker and global search results. Defaults to true.
    pub git_exclude: bool,
    /// WalkBuilder options
    /// Maximum Depth to recurse directories in file picker and global search. Defaults to `None`.
    pub max_depth: Option<usize>,
}

In cargo xtask docgen, Helix will be compiled with the #[cfg(doc_gen)]. This allows us to have documentation in just 1 place

Soon that Helix will support showing markdown syntax highlighting in documentation comments, this will make reading the documentation from source code just as good as reading it from the markdown file.