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/prelude.rs

@@ -7,16 +7,17 @@ pub use gpui::{
     WindowContext,
 };
 
-pub use crate::clickable::*;
-pub use crate::disableable::*;
-pub use crate::fixed::*;
-pub use crate::selectable::*;
 pub use crate::styles::{rems_from_px, vh, vw, PlatformStyle, StyledTypography, TextSize};
-pub use crate::visible_on_hover::*;
+pub use crate::traits::clickable::*;
+pub use crate::traits::disableable::*;
+pub use crate::traits::fixed::*;
+pub use crate::traits::selectable::*;
+pub use crate::traits::styled_ext::*;
+pub use crate::traits::visible_on_hover::*;
 pub use crate::Spacing;
 pub use crate::{h_flex, v_flex};
 pub use crate::{Button, ButtonSize, ButtonStyle, IconButton, SelectableButton};
-pub use crate::{ButtonCommon, Color, StyledExt};
+pub use crate::{ButtonCommon, Color};
 pub use crate::{Headline, HeadlineSize};
 pub use crate::{Icon, IconName, IconPosition, IconSize};
 pub use crate::{Label, LabelCommon, LabelSize, LineHeightStyle};