ui crate docs & spring cleaning (#18768)

Similar to https://github.com/zed-industries/zed/pull/18690 &
https://github.com/zed-industries/zed/pull/18695, this PR enables
required docs for `ui` and does some cleanup.

Changes:
- Enables the `deny(missing_docs)` crate-wide.
- Adds `allow(missing_docs)` on many modules until folks pick them up to
document them
- Documents some modules (all in `ui/src/styles`)
- Crate root-level organization: Traits move to `traits`, other misc
organization
- Cleaned out a bunch of unused code.

Note: I'd like to remove `utils/format_distance` but the assistant panel
uses it. To move it over to use the `time_format` crate we may need to
update it to use `time` instead of `chrono`. Needs more investigation.

Release Notes:

- N/A

crates/ui/src/ui.rs

@@ -1,28 +1,22 @@
+#![deny(missing_docs)]
+
 //! # UI – Zed UI Primitives & Components
 //!
 //! This crate provides a set of UI primitives and components that are used to build all of the elements in Zed's UI.
 //!
+//! ## Related Crates:
+//!
+//! - [`ui_macros`] - proc_macros support for this crate
+//! - [`ui_input`] - the single line input component
+//!
 
-mod clickable;
 mod components;
-mod disableable;
-mod fixed;
-mod key_bindings;
-mod path_str;
 pub mod prelude;
-mod selectable;
-mod styled_ext;
 mod styles;
+mod tests;
+mod traits;
 pub mod utils;
-mod visible_on_hover;
-mod with_rem_size;
 
-pub use clickable::*;
 pub use components::*;
-pub use disableable::*;
-pub use fixed::*;
-pub use key_bindings::*;
 pub use prelude::*;
-pub use styled_ext::*;
 pub use styles::*;
-pub use with_rem_size::*;