Shuffle player documentation, add to game event documentation

[zonical]

Partially addresses #677 and #684

This PR does two things:

• Adds a caution block to the "Features > Game Events" page to warn developers that GameEvent instances and their parameters no longer exist after the event has been fired. The new text is inserted in the section " Accessing Event Parameters" and states:

GameEvent instances and their properties will cease to exist after the event listener function is called, which means that you will encounter errors when accessing properties in timers and functions like Server.NextFrame(). You should store the value of properties in variables before calling functions like Server.NextFrame() so you can read the data safely.

• Moves the "Reference Players" page from the "Reference" section to the "Guides" section in an attempt to make the differences between player controllers and player pawns more visible and accessible to developers.

It's been a while since I've touched docfx, so please let me know if I've made any mistakes in rearranging files and the toc.yml files.