docs: Copilot offline limitations

[MarcinVaadin]

As in title

[emarc]

LGTM

[github-actions]

AI Language Review

• Limitations Section: The newly added limitation about code manipulation features not working with Vaadin offline license is not emphasized enough in terms of clarity. It may be beneficial to reword it for better clarity, such as: "Code manipulation features are incompatible with a Vaadin offline license."

• Limitations Section: The statement about AI requiring an Internet connection could be more explicit. Consider rephrasing it for precision, for example: "AI features and license validation require an Internet connection to function."

These suggestions aim to enhance clarity and ensure that users fully understand the added limitations.