Remove the todo list from the examples readme

[ealmloff]

Most of the todo list items were already added to the examples folder. For the items that haven't been added, it might be easier to track them in issues. These are the remaining items:

• Shared state example: This is covered in the state essentials guide and the guide, but we don't have any examples in the folder (Context API example #3612)
• Custom elements example (Custom element namespace example #3611)
• Testing and Debugging: This seems like a better topic for a guide on the docsite (Add a section about debugging reactivity/subscriptions docsite#374)