Document Dompdf PDFLib backend usage for performance-sensitive rendering

[Copilot]

The issue raised uncertainty about Dompdf performance after the 2.x switch and how to apply the upstream PDFLib recommendation. This PR clarifies the supported configuration path in this library and points users to the current extension distribution channel.

• Performance guidance for Dompdf backend selection

  • Added a focused “Dompdf performance” section to the README.
  • Documents that endroid/pdf forwards Dompdf options via setOptions(), including backend selection.
  • Explains that pdfBackend=PDFLib can improve performance for large/complex documents.

• Concrete configuration example

  • Added an example showing how to enable PDFLib (and optional license key) through the existing builder API:

$pdfBuilder->setOptions([
    'pdfBackend' => 'PDFLib',
    // Optional: set when your PDFLib installation requires it
    // 'pdflibLicense' => 'your-license-key',
]);

• Documentation accuracy updates

  • Updated outdated README wording that still referenced wkhtmltopdf/Snappy to reflect the current Dompdf-based implementation.
  • Added explicit reference to the pdflib extension source on PECL.

[endroid]

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