Two examples:
One in the UI section that shows how to add a tool to the toolbar along
with an icon
One in the shapes and tools section that shows a simple sticker tool
with no child states
I'll go over the copy again before it's merged, but don't want to spend
too long on it right now in case the feeling is that these should both
be a single example.
Next: The [minimal
example](https://tldraw.dev/examples/editor-api/only-editor) is
currently the best example we have of a tool with child states. I think
this should be adapted and copied/moved over to the custom shapes and
tools category.
closes tld-2266
- [x] `documentation` — Changes to the documentation only[^2]
### Release Notes
- Adds a simple custom tool example
