docs(readme): validator component is not documented

The repository has three components — Archiver, API, and Validator — but the README only documents the first two. The validator is built by 'make build' and has tests, but there is no mention of what it does, how to run it, or what configuration options it accepts. Running 'go run validator/cmd/main.go --help' shows it checks blob validity against a beacon node and blob API.

Start with the README and validator/cmd/main.go; run `make build` and `go run validator/cmd/main.go --help` to inspect the validator's behavior and configuration options. Document what the validator does, how to run it, and the accepted configuration, then verify the README accurately matches the command and existing tests.

Written by the indexing model from the issue text.