docs: add AccessKit accessibility guide (#8233)

Summary:
- add docs/accessibility.md with labels, custom widgets, and testing
guidance
- link the guide from the README accessibility FAQ

Closes #4396

Checks:
- cargo fmt --all -- --check
- git diff --check

docs/accessibility.md

@@ -0,0 +1,66 @@
+# Accessibility
+
+egui can expose its widget tree through [AccessKit](https://accesskit.dev/).
+When using `eframe`, AccessKit support is enabled by default on native
+platforms where an adapter is available. Integrations that build directly on
+`egui` can opt in with `Context::enable_accesskit()` before running frames.
+
+## Labels
+
+Most built-in widgets register their role and accessible name when shown
+through the usual egui APIs. Buttons, checkboxes, radio buttons, sliders, combo
+boxes, text edits, links, windows, images, and progress indicators all contribute
+widget information to the AccessKit tree.
+
+For inputs with a separate visible label, connect the input response to the
+label:
+
+```rust
+let label = ui.label("User name:");
+ui.text_edit_singleline(&mut user_name).labelled_by(label.id);
+```
+
+## Custom widgets
+
+If a widget is visual-only or draws custom shapes, give it explicit widget
+information:
+
+```rust
+use egui::{Sense, WidgetInfo, WidgetType};
+
+let (_rect, response) = ui.allocate_exact_size(size, Sense::click());
+// Paint using ui.painter().
+response.widget_info(|| {
+    WidgetInfo::labeled(WidgetType::Button, ui.is_enabled(), "Open color picker")
+});
+```
+
+Choose the closest `WidgetType`. For non-interactive custom content, prefer a
+label or image role where possible rather than leaving the node unnamed.
+
+## Testing
+
+`egui_kittest` builds on AccessKit, so tests can query the same roles and names
+that screen readers consume:
+
+```rust
+use egui::accesskit::Role;
+use egui_kittest::{Harness, kittest::Queryable as _};
+
+let mut harness = Harness::new_ui_state(
+    |ui, accepted| {
+        ui.checkbox(accepted, "Accept terms");
+    },
+    false,
+);
+
+harness
+    .get_by_role_and_label(Role::CheckBox, "Accept terms")
+    .click();
+harness.run();
+
+assert!(*harness.state());
+```
+
+You can also inspect the tree in the demo app when it is built with the
+`accessibility_inspector` feature.