markdown_preview: Group settings and add automatic preview opening (#63462)

## Objective

Markdown preview appearance settings are split between root theme
settings and
`markdown_preview`, and Markdown files always open as source editors
even when a user primarily
wants the rendered preview.

## Solution

- Group the Markdown preview font settings under `markdown_preview`,
migrate their legacy keys
across supported settings scopes, and update defaults, Settings UI, and
documentation.
- Add `markdown_preview.open_markdown_files_in_preview`, defaulting to
false. When enabled, clean
Markdown opens are replaced by a rendered preview while preserving dirty
editors, temporary-tab
behavior, pane activation and focus, targeted navigation, workspace
restoration, and the escape
  back to source.

## Testing

- Manually verified on macOS that enabling the setting opens Markdown
files as preview-only.
- `cargo test -p markdown_preview` (three consecutive runs)
- `cargo test -p migrator nest_markdown_preview`
- `cargo test -p settings_content`
- `cargo check -p markdown -p markdown_preview -p migrator -p settings
-p settings_content -p settings_ui -p theme_settings -p workspace`
- `./script/clippy -p gpui_apple -p markdown -p markdown_preview -p
migrator -p settings -p settings_content -p settings_ui -p
theme_settings -p workspace --features gpui_apple/runtime_shaders`
- `cargo fmt --all -- --check`
- `git diff --check`

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Recordings


https://github.com/user-attachments/assets/032ab36d-f934-4ef7-aa18-53f3955a7df8


https://github.com/user-attachments/assets/dc872265-2fe0-45ac-95eb-ff820d3a8164

Release Notes:

- Added a setting to open Markdown files directly in preview mode and
grouped Markdown preview settings together.

crates/workspace/src/workspace.rs

@@ -2733,6 +2733,15 @@ impl Workspace {
         &self.app_state
     }
 
+    pub fn is_restoring(&self) -> bool {
+        self.restoring_workspace
+    }
+
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn set_restoring_workspace(&mut self, restoring: bool) {
+        self.restoring_workspace = restoring;
+    }
+
     pub fn set_panels_task(&mut self, task: Task<Result<()>>) {
         self._panels_task = Some(task);
     }