customization: instructions for changing menus

[martinobersteiner]

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Description

Adds instructions for customizing menus.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• [x] I'm aware of the code of conduct.
• [x] I've created logical separate commits and followed the commit message format.
• [x] I've added relevant test cases.
• [x] I've added relevant documentation.
• [x] I've marked translation strings (for relevant code).
• [x] I've followed the CSS/JS and React guidelines (for relevant code).
• [x] I've followed the web accessibility guidelines (for relevant code).
• [x] I've followed the user interface guidelines (for relevant code).
• [x] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [x] I've NOT included third-party code (copy/pasted source code or new dependencies).

Third-party code

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