Update documentation of plutus-benchmark

[basetunnel]

Update some docs of plutus-benchmark

• Explain why .flat files of validation cannot be regenerated from source
• Explain the purpose of nofib and of validation benchmarks
• Add Note about how they used to be generated
• Remove comment about stack behaviour that is not relevant for cabal's benchmark behaviour

[kwxm]

Thanks for doing this: the README files can easily get out of date without anyone noticing. It's good to have a record of the history so we don;t forget it as people come and go.