Add TeamSync PDF Viewer to README - #1168

Checklist:

• I've read and understood Contributing Guidelines.
• I've added the new resource at the end of its section.
• This resource is out there for a while, and actively maintained.
• This resource is popular enough and has at least a few hundred stars on GitHub.

Adding TeamSync PDF Viewer to the Files section, alongside jsPDF and PDF.js.

It's an open-source PDF viewer SDK for React. Where jsPDF generates PDFs and PDF.js renders them, TeamSync covers working on them - annotation, redaction, watermarking and search - and it all runs client-side with no server component.

Two things that seemed worth a line in this list:

• Redaction is applied at binary level with the text layer sanitised, rather than drawing a rectangle over the content
• Annotations export as XFDF, so a file annotated in the browser opens correctly in Acrobat

Built on React 19 and TypeScript. CPAL-1.0. 246 stars, published on npm as teamsync-pdf-viewer, live demo at pdfviewer.teamsync.com.

Disclosure: I work with the project.