Add example to automatically sign when the document is loaded

[irgendeinich]

Details

Adds another example to our "Native Catalog" showing how to apply a Watermark as soon as a document URL is set.

Acceptance Criteria

• When approved, right before merging, rebase with master and increment the package version in package.json, package-lock.json, samples/Catalog/package.json, and samples/NativeCatalog/package.json (see example commit: 1bf805f).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/react-native/releases).