Add general dropdowns to main menu

[adlerzei]

• Fixes General purpose drop down menu in navbar #311 and fixes General purpose drop down menu in Docsy #1673: dropdown menu items can be added.
• Adds general dropdown menu items to the top-level menu.
  • Active submenu items are highlighted when they are open or when a subordinate page is open.
  • The items can be added to a dropdown using the parent parameter in front matter.
• Adds documentation for the new dropdown menu items.

Screenshots

Mobile / narrow

Closed	Open

Tablet

Desktop