docs: describe Edit PDF annotations and forms

[lKolabrodl]

Closes #71.

Summary

• document Edit PDF notes, highlights, underlines, strikeouts, and ink annotations
• document supported AcroForm text, checkbox, radio, combo, and list fields
• explain the different scopes of form-field and annotation flattening
• state the current XFA, multi-file form, and source-content editing limits
• update the roadmap to identify v0.3.1 as the current published release

Why

The public README and roadmap still described Edit PDF as an overlay-only text, image, and shape editor. That no longer reflected the annotation and AcroForm workflows available on the development branch, and the roadmap still presented the already-published v0.3.1 release as upcoming.

Validation

• npm run build
• npm test - 30 files and 261 tests passed
• cargo check --manifest-path src-tauri/Cargo.toml - not applicable; documentation only

Privacy Checklist

• This keeps OffPDF usable offline.
• This does not upload, log, or transmit user files.
• No dependencies or bundled binaries were added.

AI disclosure

AI assistance was used to cross-check the documentation against the editor types, form controls, flattening implementation, and published release metadata. The wording and limits were then reviewed against the source and verified with the commands above.

[McanKul]

Thanks for the clear documentation update and for keeping the scope aligned with the current implementation. Great first contribution!