Is your feature request related to a problem? Please describe.

I'm always frustrated when we're not grouping our settings clearly and have excessive top level configuration.

Describe the solution you'd like

1. Move without_mainnet into mainnet.disabled, same meaning, same default, keep the old one for a release (compatibility).
2. Move governance.disable into mainnet.disable_governance_sync, same behavior, as above.
3. Move fee.main_chain into mainnet.extra_fee, same behavior, as above.
4. Move contracts into mainnet.contracts, same behavior (just two contracts there), as above.

Describe alternatives you've considered

Keeping things as is, works too.

Additional context

#2423, #3361.