Add Default Cardano developer environment how-to guide

## 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)
- #115 (missing onboarding content)
- #201 (pathway gaps; this is a smaller concrete slice)

Start by reviewing the existing content under website/docs/how-to-guide/, the Getting Started page, the beginner README, and the related Session 17 notes. Draft the newcomer-focused default environment guide, link it from the requested entry points, and run npm run build. Done means the page is published, links are present, steps cover the stated success checks and graduation path, and the build succeeds.

Written by the indexing model from the issue text.