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/assistant/src/assistant_panel.rs

@@ -77,7 +77,7 @@ use ui::TintColor;
 use ui::{
     prelude::*,
     utils::{format_distance_from_now, DateTimeType},
-    Avatar, AvatarShape, ButtonLike, ContextMenu, Disclosure, ElevationIndex, KeyBinding, ListItem,
+    Avatar, ButtonLike, ContextMenu, Disclosure, ElevationIndex, KeyBinding, ListItem,
     ListItemSpacing, PopoverMenu, PopoverMenuHandle, Tooltip,
 };
 use util::{maybe, ResultExt};
@@ -262,9 +262,7 @@ impl PickerDelegate for SavedContextPickerDelegate {
                             .gap_2()
                             .children(if let Some(host_user) = host_user {
                                 vec![
-                                    Avatar::new(host_user.avatar_uri.clone())
-                                        .shape(AvatarShape::Circle)
-                                        .into_any_element(),
+                                    Avatar::new(host_user.avatar_uri.clone()).into_any_element(),
                                     Label::new(format!("Shared by @{}", host_user.github_login))
                                         .color(Color::Muted)
                                         .size(LabelSize::Small)