docs: add S3 native locking docs and fix variable descriptions (#197)

- Fix s3_state_lock_enabled description (was incorrectly 'Whether to
  create the S3 bucket', now describes S3 native state locking)
- Add deprecation note to dynamodb_enabled description with link to
  official Terraform docs
- Add 'S3 Native State Locking' section to README with usage example
  showing s3_state_lock_enabled=true and dynamodb_enabled=false

Ref: https://developer.hashicorp.com/terraform/language/backend/s3#s3-state-locking
Ref: https://developer.hashicorp.com/terraform/language/backend/s3#enabling-dynamodb-state-locking-deprecated
Closes #188
Closes #194