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/components/context_menu.rs

@@ -1,6 +1,7 @@
+#![allow(missing_docs)]
 use crate::{
-    h_flex, prelude::*, v_flex, Icon, IconName, KeyBinding, Label, List, ListItem, ListSeparator,
-    ListSubHeader, WithRemSize,
+    h_flex, prelude::*, utils::WithRemSize, v_flex, Icon, IconName, KeyBinding, Label, List,
+    ListItem, ListSeparator, ListSubHeader,
 };
 use gpui::{
     px, Action, AnyElement, AppContext, DismissEvent, EventEmitter, FocusHandle, FocusableView,