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)

Beginne damit, die bestehenden Inhalte unter website/docs/how-to-guide/, die Getting Started-Seite, das Anfänger-README und die zugehörigen Notizen zu Session 17 zu prüfen. Erstelle den auf Neueinsteiger ausgerichteten Leitfaden für die Standardumgebung, verlinke ihn von den angeforderten Einstiegspunkten und führe npm run build aus. Erledigt bedeutet, dass die Seite veröffentlicht ist, die Links vorhanden sind, die Schritte die angegebenen Erfolgskriterien und den weiteren Entwicklungsweg abdecken und der Build erfolgreich ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.