Consolidate special key tutorials

[dphfox]

As Fusion has evolved, pretty much all generic functionality has moved out of special keys, leaving behind a string of special keys which are pretty simple to explain as they target very specific use cases in the Roblox API. As a result, these keys take up more space in docs, which is less justifiable now that they play a diminished role relative to the rest of the library design.

I propose looking into ways of presenting special keys as a single collection of features achieving a common goal rather than having multiple drawn out tutorial pages that repeat a lot of information for each special key type. It'd be good to motivate the idea of special keys as a whole, and then break down how each special key targets a different segment of the Roblox API.