Add Default Cardano developer environment how-to guide

[danbaruka]

Problem

Newcomers still lack a durable, opinionated default local developer environment guide on the DevEx site. Working Group Session 17 discussed local vs public testnets and tools like Yaci DevKit, but that content mostly lives in session notes rather than a first-class how-to.

The Intermediate how-to section was empty and removed (#191), which widens this gap between Beginner and Advanced.

Proposed solution

Publish a practical how-to under website/docs/how-to-guide/ that covers:

1. Recommended default path for new builders (local-first where possible)
2. Setup steps (e.g. Yaci DevKit / local devnet + indexer options)
3. First success checks (query tip, faucet/test ADA, simple submit path)
4. Graduation path to preview/preprod/mainnet
5. Links out to Developer Portal / Mesh / Blockfrost as needed

Link the guide from Getting Started and Beginner README.

Acceptance criteria

• New how-to page published (not only WG session notes)
• Linked from Getting Started (and beginner index where appropriate)
• Written for newcomers (clear steps, minimal jargon)
• npm run build succeeds

Related

• Session 17 notes (Default Developer Environment)
• Missing content on DevEx site #115 (missing onboarding content)
• Core contributor pathway is underspecified — gaps, missing resources, and structural feedback #201 (pathway gaps; this is a smaller concrete slice)