Document smart contract validator management + general validator how-to

[alexandratran]

Describe the bug

• [x] Missing content
• [ ] Outdated content
• [ ] Wrong content
• [ ] Confusing or misleading content
• [ ] Other

Change suggestion

Add section on adding/removing validators via smart contract to the Configure QBFT page, similar to Besu.

Also add general information to this page on how to add/remove validators via block header voting (similar to Besu) - but simplified, linking to the existing tutorial. Also add this general info to the Configure IBFT page.

First confirm that smart contract validator management is implemented in GoQuorum.

[rolandtyler]

smart contract validators merged here