Added low brightness mode, added speed mode, more documentation, added fixes from KarFreitag

[cRoCx]

Since I've got it working a while ago I would like to contribute a few things.

• I've added a low brightness mode which is good in low lit environments in the evening at home.
• I've added a high speed mode, which just increases the base game speed from 0.2 to 0.6. All other speed calculations remain unaffected.
• Both modes can be enabled by holding down either player button just before the game starts. Both modes can be enabled at the same time. No additional buttons needed.
• I've changed the behaviour of the standby animations which start after 30s of inactivity. They no longer change every 5s, but in much slower intervals. This is less attention seeking in high traffic environments, thus maybe worse there, but more cozy at home or at a chillout evening with friends.
• I've merged all other features and bugfixes by @KarFreitag, except his PlatformIO refactoring.
• Unfortunately I haven't tested, how the "Start" button affects anything. I don't use the start button. I use only the two player buttons and reset the game by powering off and on the Arduino.
• I've added more documentation from my previously opened issues and a general game description.