chore: Update Copilot docs after removing Command Window, align with V25

[MarcinVaadin]

As in title.

Fixes: https://github.com/vaadin/copilot-internal/issues/6875

Using main as target branch as change is only for v25

[github-actions]

AI Language Review

• In the "Copilot" document, under the "Copilot Trial" section, the updated text mentions a "seven-day trial," whereas the previous version mentioned a "three-day preview" and a "30-day trial period." Ensure these details are consistent with actual offerings.

• In the "Vaadin Copilot UI" section, the document now mentions "four main parts" instead of "five main parts." Ensure this change accurately reflects the current UI structure.

• In the "Example Prompts" section, the example prompt was changed from "make the button primary" to "make this button primary." Verify that this change still conveys the intended action accurately.

• In the "Figma to Vaadin" section, a link to the Eclipse plugin has been added. Verify the new link to the Vaadin Tools on the Eclipse Marketplace to ensure it works correctly.

Overall, these changes should be confirmed with the development team or in accordance with current product specifications to ensure the documentation remains accurate and up-to-date.