Document endpoint required by Copilot #4427
Description
Should be added to https://vaadin.com/docs/latest/tools/copilot (maybe a subsection):
- Mention Copilot loads external resources that are not bundled with the framework, as this will cause errors for users with custom CSP headers or tighter firewall configs.
- Explicitly document all required endpoints so they can be included/whitelisted as needed.
- Ideally, include a brief description of the "purpose" of each endpoint.