WIP: Add experimental --Xsnapsync-bft-enabled option

[joaniefromtheblock]

This is still being developed. Check Eng PR to get status on feature

• Update docs to reflect "how do the different sync modes work" and add private syncing to https://besu.hyperledger.org/development/public-networks/get-started/connect/sync-node

• Document --Xsnapsync-bft-enabled which enables snap sync for BFT chains.

Related to: https://github.com/hyperledger/besu/pull/7140/commits

[alexandratran]

Note: We don't typically document experimental options in the CLI reference, but they can be mentioned in other parts of the docs.

[joaniefromtheblock]

Working on this. Need to get some more clarity from Eng. Will pick up next week.