Replace notion of "Chain" with notion of "Instance"

[nbundi]

currently we only support grouping reviews of different chain deployments by protocol

more generally, we need to account for different "Instances" of a protocol

eg Aave has a Main instance, Lido instance and Etherfi instance on Ethereum. Then it also has instances on different chains

we need to account for all these different instances, some are on the same chain, some on different chains

so also rename the column name "Chain" to "Instance"