[EUI+] Rethink and update fullscreen demos

[weronikaolejniczak]

The purpose of this task is to rethink and update the fullscreen demos. Docs with fullscreen demos (EuiHeader, EuiTour) need to be rethought or excluded for now as they are completely broken due to the custom behavior.

---

Pages affected:

• [ ] EuiHeader’s Elastic navigation pattern example - it likely needs to be its own page / separate from the main examples, as it’s automatically taking over the docusaurus header (accessibility issue),
• [ ] EuiTour’s fullscreen demo - also broken / errors with “require is not defined”,
• [ ] EuiCollapsibleNav’s putting it all together example - errors with “require is not defined”.