What is the purpose of the small, round knob/button in the triangular cutout towards the back of the bolt body (just in front of the bolt handle/rear baffle)?

This small button is called the cocking piece pin. Its primary purpose is to control the forward movement of the firing pin assembly.  As the bolt handle is locked down into firing position, the cocking piece pin moves to the center of the triangular cutout and is held in place by the sear.  When the trigger is pulled, the cocking piece pin snaps forward releasing the firing pin to strike the primer/rim of the cartridge in the chamber – discharging the firearm.  When the bolt handle is lifted, the cocking piece pin moves back to its original position – cocking the firearm. If the COCKING PIECE PIN is deliberately forced out of place while the bolt is partially open, the firing pin may be able to strike the primer/rim of the cartridge – firing the cartridge.  Detailed in the INSTRUCTION MANUAL, there are clearly defined steps for the safe operation of your firearm, none of which requiring the COCKING PIECE PIN to be touched, moved, or in any way pushed out of position.  As with any firearm, physically moving or forcing any component out of its designed position may result in damage, injury, or even death.