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/welcome/src/base_keymap_picker.rs

@@ -12,7 +12,13 @@ use ui::{ListItem, ListItemSpacing, prelude::*};
 use util::ResultExt;
 use workspace::{ModalView, Workspace, ui::HighlightedLabel};
 
-actions!(welcome, [ToggleBaseKeymapSelector]);
+actions!(
+    welcome,
+    [
+        /// Toggles the base keymap selector modal.
+        ToggleBaseKeymapSelector
+    ]
+);
 
 pub fn init(cx: &mut App) {
     cx.observe_new(|workspace: &mut Workspace, _window, _cx| {

crates/welcome/src/welcome.rs

@@ -25,7 +25,13 @@ mod base_keymap_setting;
 mod multibuffer_hint;
 mod welcome_ui;
 
-actions!(welcome, [ResetHints]);
+actions!(
+    welcome,
+    [
+        /// Resets the welcome screen hints to their initial state.
+        ResetHints
+    ]
+);
 
 pub const FIRST_OPEN: &str = "first_open";
 pub const DOCS_URL: &str = "https://zed.dev/docs/";