Add Validator Rewards/Incentives to Handbook

[chrlschwb]

Currently the Joystream Handbook is more like a technical document. It doesn't contain information that is vital to some users.
One of these is information about the rewards that validators get.

https://handbook.joystream.org/system/validation

This polkadot wiki page could be used as inspiration.

https://wiki.polkadot.network/docs/maintain-guides-validator-payout

[traumschule]

This is mostly completed except for Advanced Setup at the bottom. It doesn't hurt however to get another pair of eyes and verify the existing page.