Documentation of mk_arcade_joystick_rpi GPIO module

[Lolonois]

I noticed that the mk_arcade_joystick_rpi GPIO module is not listed at [1] but only appears in an appendix for the RPI4 at [2]. I guess this versatile module should be placed where it can easily be found in the documentation.

Sure, it was broken [3] in the past for the RPI4 and from kernel 4.15 onwards, but those issues are fixed and I can confirm this module works well with a RPI3 and RPI4 (both branches retropie and retropie-hotkeybtn) on latest RetroPie.

Is the absence on [1] because nobody did document it yet for being on par with the other GPIO modules? In that case I can provide some updates to the page.

Thanks for sharing some insights.

[1] https://github.com/RetroPie/RetroPie-Docs/blob/master/docs/GPIO-Modules.md
[2] https://github.com/RetroPie/RetroPie-Docs/blob/master/docs/Pi4.md#retropie-changes-for--raspberry-pi-4
[3] https://github.com/cmitu/mk_arcade_joystick_rpi/pull/1, https://github.com/cmitu/mk_arcade_joystick_rpi/pull/2