Annotate shape meta data example (#2453)

Annotate shape meta data example

### Change Type

- [ ] `patch` — Bug fix
- [ ] `minor` — New feature
- [ ] `major` — Breaking change
- [ ] `dependencies` — Changes to package dependencies[^1]
- [x] `documentation` — Changes to the documentation only[^2]
- [ ] `tests` — Changes to any test code only[^2]
- [ ] `internal` — Any other changes that don't affect the published
package[^2]
- [ ] I don't know

[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version

### Test Plan

1. Add a step-by-step description of how to test your PR here.
2.

- [ ] Unit Tests
- [ ] End to end tests

### Release Notes

- Annotate shape meta data example

---------

Co-authored-by: Steve Ruiz <steveruizok@gmail.com>

apps/examples/src/examples/shape-meta/ShapeMetaExample.tsx

@@ -1,6 +1,9 @@
 import { TLShape, Tldraw, track, useEditor } from '@tldraw/tldraw'
 import '@tldraw/tldraw/tldraw.css'
 
+// There's a guide at the bottom of this file!
+
+// [1]
 export default function ShapeMetaExample() {
 	return (
 		<div className="tldraw__editor">
@@ -18,10 +21,10 @@ export default function ShapeMetaExample() {
 	)
 }
 
-// By default, the TLShape type's meta property is { [key: string]: any }, but we can type it
-// by unioning the type with a new type that has a meta property of our choosing.
+// [2]
 type ShapeWithMyMeta = TLShape & { meta: { label: string } }
 
+// [3]
 export const ShapeLabelUiWithHelper = track(function ShapeLabelUiWithHelper() {
 	const editor = useEditor()
 	const onlySelectedShape = editor.getOnlySelectedShape() as ShapeWithMyMeta | null
@@ -47,3 +50,30 @@ export const ShapeLabelUiWithHelper = track(function ShapeLabelUiWithHelper() {
 		</div>
 	)
 })
+
+/* 
+This example shows how to use the `getInitialMetaForShape` function to set initial
+meta data for a shape and update it.
+
+[1]
+In the Tldraw component's `onMount` callback, we override the default 
+`Editor.getInitialMetaForShape` function. This function is called when
+a new shape is created and provides the `meta` property value for the 
+new shape.
+
+[2]
+By default, the TLShape type's meta property is { [key: string]: any }, but we can type
+it using a union like this.
+
+[3]
+This is our minimal ui for editing the meta data of our shapes. We use the `track` 
+function to make sure this component is reactive, it will re-render when the signals
+it is tracking change. Check out the signia docs for more: 
+https://signia.tldraw.dev/docs/API/signia_react/functions/track
+
+Because our component is a child of the Tldraw component, it has access to the `useEditor` 
+hook via React context. We use this to get the only selected shape. If there is a single 
+shape selected, we render the input and update the shape's meta data when the input changes 
+via the `onChange` function.
+
+*/