Z-index is not correct for sticky ic-page-header

@MI6-806 y travaille déjà.

Depuis le 9/6/2023.

## Summary of the bug
Spotted on the guidance site, the page header is sticky and should be shown above any page content. However, the ic-menu is displayed over it when it is opened and the page is scrolled.

## 🪜 How to reproduce
1. Go to https://design.sis.gov.uk/components/select.
2. Click the select example on the guidance tab so that the menu is open.
3. Scroll down whilst keeping the menu open.
4. The menu will appear above the sticky page header.

## 🧐 Expected behaviour
A sticky page header should cover all page content.

## 📸 Screenshots or code

## Additional info
Will maybe need a new design token for a sticky page header z-index. Could then add that to page header when the sticky prop is applied.