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/vim/src/normal/paste.rs

@@ -14,6 +14,7 @@ use crate::{
     state::{Mode, Register},
 };
 
+/// Pastes text from the specified register at the cursor position.
 #[derive(Clone, Deserialize, JsonSchema, PartialEq, Action)]
 #[action(namespace = vim)]
 #[serde(deny_unknown_fields)]