[MenuBar] Make menu start index more consistent.

[bruvzg]

• Removes unrelated MenuButton links for docs.
• Adds few extra notes to the docs.
• Makes start_index work as sort order. Previously MenuBar was using it as an actual index of menu item in the main menu, but since MenuBar can have multiple items and always is a continuous item range it was counterintuitive and hard to use. Now menu is simply added before first menu with the higher or equal start_index (or at the end if it's -1).

[akien-mga]

Thanks!