Add documentation for custom crosshairs

[ninjrdevelop]

Following on from #71, add documentation (somewhere). Initial notes here: #72

[ninjrdevelop]

Step 1) Created a crosshair generation page: https://ninjrdevelop.github.io/sandbox-plus-crosshair/
DONE

Step 2) Write actual documentation
Pending

Step 3) Add documentation and generator to the crosshair panel.