Docs/onboarding updates

[alltheseas]

Summary

• add repo-level getting-started instructions (toolchain, just
  targets, examples, offline docs) plus context for the Supported
  NIPs table
• flesh out crate-level READMEs (relay pool/builder, keyring,
  gossip engines, RFS) with descriptions, feature notes, and runnable
  examples so integrators know how to use them
• expand contributor docs with workflow/script guidance and spell
  out the private vulnerability reporting process

Testing

• documentation-only changes

[alltheseas]

@yukibtc I found a painpoint of pretty much all FOSS repos not having comprehensive documentation, as the code is more important. I am not a rust-nostr expert, so I look to your sanity check on this documentation 🙏

[yukibtc]

Thanks @alltheseas! I've cherry-picked some of the changes.

Regarding the NIP-09 example: I prefer to put specific examples regarding NIPs or certain SDK features in the book, so that correctness can always be verified by CI during various updates and examples can also be written for bindings.
So I moved your example here: nostrdevkit/book#19