Add more information to the Menu guide

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- Mention how to check for permissions and that they need to check for permissions in Menu and for example in the Controller as well as it would be otherwise still possible to execute the controller via URL
- Menus are often needed in combination with Pages therefore we should mention Pages (Controllers) and link to them
- How to rename Menu items
- How to delete Menu items (I think for both delete & rename there is a `configure` method in the Menu class otherwise we need to explain the events)
- Explain that they need to find the translation key to rename/delete menu items and how they can find it (switch language to Development or under Admin => Translation search)
- Update Admin and User menu screen
- Explain at the beginning what Menus are good for (restrict items to certain users / user-groups etc, remove unneeded items etc) refs #80

Review the Menu guide, including the Admin and User menu screens, and trace the mentioned Menu class configure method plus the Pages/Controllers and translation-search references. Update the guide to cover permissions, related pages, renaming and deleting items, translation-key discovery, and the purpose of menus; done means each requested topic is documented and the relevant links are present.

Written by the indexing model from the issue text.