docs: Add Context & ElementId doc. (#1460)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

docs/.gitignore

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 .vitepress/cache
 .vitepress/dist
+bun.lockb

docs/docs/context.md

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+---
+title: Context
+description: Learn about the Window and Context in GPUI.
+order: -4
+---
+
+The [Window], [App], [Context] and [Entity] are most important things in GPUI, it appears everywhere.
+
+- [Window] - The current window instance, which for handle the **Window Level** things.
+- [App] - The current application instance, which for handle the **Application Level** things.
+- [Context] - The Entity Context instance, which for handle the **Context Level** things.
+- [Entity] - The Entity instance, which for handle the **Entity Level** things.
+
+For example:
+
+```rs
+fn new(window: &mut Window, cx: &mut App) {}
+
+impl RenderOnce for MyElement {
+    fn render(self, window: &mut Window, cx: &mut App) {}
+}
+
+impl Render for MyView {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) {}
+}
+```
+
+:::info
+As you can see, we always use `cx` to represent `App` and `Context<Self>`,
+which is the standard naming convention for GPUI,
+we can follow this convention to make our code more readable and maintainable.
+:::
+
+[Window]: https://docs.rs/gpui/latest/gpui/struct.Window.html
+[App]: https://docs.rs/gpui/latest/gpui/struct.App.html
+[Context]: https://docs.rs/gpui/latest/gpui/struct.Context.html
+[Entity]: https://docs.rs/gpui/latest/gpui/struct.Entity.html

docs/docs/element_id.md

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+---
+title: ElementId
+description: To introduce the ElementId concept in GPUI.
+order: -4
+---
+
+The [ElementId] is a unique identifier for a GPUI element. It is used to reference elements in the GPUI component tree.
+
+Before you start using GPUI and GPUI Component, you need to understand the [ElementId].
+
+For example:
+
+```rs
+div().id("my-element").child("Hello, World!")
+```
+
+In this case, the `div` element has an `id` of `"my-element"`. The add `id` is used for GPUI for binding events, for example `on_click` or `on_mouse_move`, the `element` with `id` in GPUI we call [Stateful\<E\>].
+
+We also use `id` (actually, it uses [GlobalElementId] internally in GPUI) to manage the `state` in some elements, by using `window.use_keyed_state`, so it is important to keep the `id` unique.
+
+## Unique
+
+The `id` should be unique within the layout scope (In a same [Stateful\<E\>] parent).
+
+For example we have a list with multiple items:
+
+```rs
+div().id("app").child(
+    div().id("list1").child(vec![
+        div().id(1).child("Item 1"),
+        div().id(2).child("Item 2"),
+        div().id(3).child("Item 3"),
+    ])
+).child(
+    div().id("list2").child(vec![
+        div().id(1).child("Item 1"),
+    ])
+)
+```
+
+In this case, we can named the child items with a very simple id, because they are have a parent `list1` element with an `id`.
+
+GPUI internal will generate [GlobalElementId] with the parent elements's `id`, in this example, the `Item 1` will have global_id:
+
+```rs
+["app", "list1", 1]
+```
+
+And the `Item 1` in `list2` will have global_id:
+
+```rs
+["app", "list2", 1]
+```
+
+So we can named the child items with a very simple id.
+
+[ElementId]: https://docs.rs/gpui/latest/gpui/enum.ElementId.html
+[GlobalElementId]: https://docs.rs/gpui/latest/gpui/struct.GlobalElementId.html
+[Stateful]: https://docs.rs/gpui/latest/gpui/struct.Stateful.html
+[Stateful\<E\>]: https://docs.rs/gpui/latest/gpui/struct.Stateful.html

docs/docs/root.md

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 ---
-order: -3
+order: -7
 ---
 
 # Root View