Run doctests in CI and fix up existing doctests (#37851)

Follows on from
https://github.com/zed-industries/zed/pull/37716#pullrequestreview-3195695110
by @SomeoneToIgnore

After this the doctests will be run in CI to check that the examples are
still accurate.

Note that doctests aren't run by Nextest: you can run them locally with
`cargo test --doc`.

Summary:
* Run tests from CI
* Loosen an exact float comparison to match approximately (otherwise it
fails)
* Fixed one actual bug in the tests for `dilate` where the test code
assumed that `dilate` mutates `self` rather than returning a new object
* Add some `must_use` on some functions that seemed at risk of similar
bugs, following the Rust stdlib style to add it where ignoring the
result is almost certainly a bug.
* Fix some cases where the doc examples seem to have gone out of date
with the code
* Add imports to doctests that need them
* Add some dev-dependencies to make the tests build
* Fix the `key_dispatch` module docstring, which was accidentally
attached to objects within that module
* Skip some doctest examples that seem like they need an async
environment or that just looked hard to get running

AI usage: I asked Claude to do some of the repetitive tests. I checked
the output and fixed up some things that seemed to not be in the right
spirit of the test, or too longwinded.

I think we could reasonably run the tests on only Linux to save CI
CPU-seconds and latency, but I haven't done that yet, partly because of
how it's implemented in the action.

Release Notes:

- N/A

crates/ui/src/components/button/button.rs

@@ -43,7 +43,7 @@ use super::button_icon::ButtonIcon;
 ///
 /// Button::new("button_id", "Click me!")
 ///     .icon(IconName::Check)
-///     .selected(true)
+///     .toggle_state(true)
 ///     .on_click(|event, window, cx| {
 ///         // Handle click event
 ///     });
@@ -56,7 +56,7 @@ use super::button_icon::ButtonIcon;
 /// use ui::TintColor;
 ///
 /// Button::new("button_id", "Click me!")
-///     .selected(true)
+///     .toggle_state(true)
 ///     .selected_style(ButtonStyle::Tinted(TintColor::Accent))
 ///     .on_click(|event, window, cx| {
 ///         // Handle click event
@@ -228,7 +228,7 @@ impl Toggleable for Button {
     /// use ui::prelude::*;
     ///
     /// Button::new("button_id", "Click me!")
-    ///     .selected(true)
+    ///     .toggle_state(true)
     ///     .on_click(|event, window, cx| {
     ///         // Handle click event
     ///     });
@@ -251,7 +251,7 @@ impl SelectableButton for Button {
     /// use ui::TintColor;
     ///
     /// Button::new("button_id", "Click me!")
-    ///     .selected(true)
+    ///     .toggle_state(true)
     ///     .selected_style(ButtonStyle::Tinted(TintColor::Accent))
     ///     .on_click(|event, window, cx| {
     ///         // Handle click event
@@ -317,7 +317,7 @@ impl FixedWidth for Button {
     /// use ui::prelude::*;
     ///
     /// Button::new("button_id", "Click me!")
-    ///     .width(px(100.).into())
+    ///     .width(px(100.))
     ///     .on_click(|event, window, cx| {
     ///         // Handle click event
     ///     });
@@ -381,7 +381,7 @@ impl ButtonCommon for Button {
     /// use ui::Tooltip;
     ///
     /// Button::new("button_id", "Click me!")
-    ///     .tooltip(Tooltip::text_f("This is a tooltip", cx))
+    ///     .tooltip(Tooltip::text("This is a tooltip"))
     ///     .on_click(|event, window, cx| {
     ///         // Handle click event
     ///     });