docs: add development section on action menus

[Shrinks99]

Closes #2428

Caveats

• Not all of the dropdowns in the app follow these rules yet (though as of Add "Copy <item type> ID" to a bunch of menus #2426 we're closer!)
• Might be cool to have some images?

[SuaYoo]

Looks good, I do feel like this shouldn't be public facing in top-level navigation until there's a landing page for the design guide. It feels like too much importance placed on action menus. Maybe we take out of the navigation for now and pin the page to the Discord design channel? And/or add a simple "Design" index page that says clearly that it's a work in progress?