gpui: Store action documentation (#33809)

Closes #ISSUE

Adds a new `documentation` method to actions, that is extracted from doc
comments when using the `actions!` or derive macros.

Additionally, this PR adds doc comments to as many action definitions in
Zed as possible.

Release Notes:

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

crates/markdown/src/markdown.rs

@@ -141,7 +141,15 @@ pub type CodeBlockRenderFn = Arc<
 pub type CodeBlockTransformFn =
     Arc<dyn Fn(AnyDiv, Range<usize>, CodeBlockMetadata, &mut Window, &App) -> AnyDiv>;
 
-actions!(markdown, [Copy, CopyAsMarkdown]);
+actions!(
+    markdown,
+    [
+        /// Copies the selected text to the clipboard.
+        Copy,
+        /// Copies the selected text as markdown to the clipboard.
+        CopyAsMarkdown
+    ]
+);
 
 impl Markdown {
     pub fn new(