docs: tutorial — write your own decoder in 15 minutes

[Omoboi-dev]

Context
Decoders are the #1 way to contribute. A friendly tutorial turns curious
visitors into contributors.

Task
Write a step-by-step tutorial that walks a newcomer through adding a decoder,
using the example TokenDecoder, ending with a passing test.

Pointers

• Add to docs/; link from README + CONTRIBUTING.
• Reference ingestor/crates/decoders/src/lib.rs.

Acceptance criteria

• A complete, accurate walkthrough from zero to a tested decoder
• Linked from README + CONTRIBUTING
• Beginner-friendly tone

Notes
🟢 Good first issue and high-leverage for the whole project.