Added docs page for I.K.E.M.E.N GO

[SuperFromND]

Note: This should not be merged until RetroPie/RetroPie-Setup/pull/3384 (adding IKEMEN GO to RetroPie as a scriptmodule) is merged!

Decided to write some basic documentation about configuring I.K.E.M.E.N GO while waiting for the above PR to get resolved/merged. This isn't an all-encompassing guide to customizing every part of the engine (as that would be way too long for a single RetroPie docs page and is probably best left to IKEMEN's developers and community), but the one I wrote should at least be able to help newcomers install stuff like characters and stages.

Feel free to add any other information I might have missed!