Add/update stellar events public docs

[chowbao]

With the release of stellar events (previously known as unified events) in P23 we need to update some of our public docs

• Update core concepts --> smart contract --> events
  • Could be moved to core concepts --> stellar data structures as it is not a smart contract only structure anymore
• Update how to guide --> contract events to stellar events
• Add a retroactive events usage/backfill page either as a how to guide or somewhere
• In general search replace contract events --> stellar events