Add a tutorial page for HDR in momentum maps

[hexaflexahexagon]

We may want to add a mapping tutorial on our docs page explaining hdr, what's different between old games and momentum (e.g. it's forced on, no LDR), how it works, how to use it (specific entities like the tonemap controller), and maybe suggestions for where to use it relative to movement gamemodes since it's a different use case to traditional, slow single player games.

related: momentum-mod/game#1334