docs: add usage examples to README and bump to v0.5.1

[Copilot]

README lacked practical usage examples. API Reference only documented one method despite the toolkit exposing invoice/credit note generation, XML parsing, totals computation, and EAS lookup.

README changes

• Quick Start — stripped placeholder {} examples; replaced with minimal working ESM/CJS imports
• Usage Examples (new section) — six end-to-end snippets covering:
  • invoiceToPeppolUBL / creditNoteToPeppolUBL with full typed payloads
  • peppolUBLToInvoice / peppolUBLToCreditNote round-trip parsing
  • PeppolToolkit.computeTotals() with multi-rate VAT breakdown
  • PeppolToolkit.getEASFromTaxId() country-prefix → EAS scheme lookup
  • exampleInvoice / exampleCreditNote built-in fixtures
• API Reference — replaced prose bullets with method/helper tables covering all public surface
• Roadmap — marked completed items (invoice API, types, validation helpers, CreditNote, docs)

Version

• package.json: 0.5.0 → 0.5.1

Example: compute totals across mixed VAT rates

import { PeppolToolkit } from '@pixeldrive/peppol-toolkit';

const totals = PeppolToolkit.computeTotals([
    { price: 100,   quantity: 10, taxPercent: 21 },
    { price: 49.99, quantity: 3,  taxPercent: 21 },
    { price: 200,   quantity: 2,  taxPercent: 0  },
]);

console.log(totals.baseAmount.toNumber());   // 1549.97
console.log(totals.taxAmount.toNumber());    // 241.49
console.log(totals.totalAmount.toNumber());  // 1791.46

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.