Document addition and removal of BFT nodes from channel config

There is currently no documentation that explains how to edit the channel config to add a new orderer node and remove it.

For BFT, the following things need to be taken care of:

- Add the orderer to the [consenter mapping](https://github.com/hyperledger/fabric/blob/main/sampleconfig/configtx.yaml#L324-L352)
- Add the orderer's endpoint [to the endpoints](https://github.com/hyperledger/fabric/blob/main/sampleconfig/configtx.yaml#L75-L76).
- Adjust the [block validation policy](https://github.com/hyperledger/fabric/blob/main/sampleconfig/configtx.yaml#L418-L422) accordingly.

Similarly, for removal.