Add "prefix" to PDF exporter

[blwsk]

Is your feature request related to a problem? Please describe.
Status quo of the PDF exporter class supports optional header and footer parameters that are applied to each exported page. It would be useful to include an additional parameter, prefix, that adds the specified context only to the beginning of the exported document.

Describe the solution you'd like
It would be useful to include an additional parameter, prefer, that adds the specified context only to the beginning of the exported document. I.e.:

import { Text } from "@react-pdf/renderer";
const pdfDocument = await exporter.toReactPDFDocument(editor.document, {
  prefix: <Text>Prefix. Only appearing at the top of the first page, below the header.</Text>,
});

Describe alternatives you've considered
A current workaround is to hand write one or several blocks that don't exist in the editor's document and concatenate them at the beginning of editor.document. A big drawback to this approach is that any content you want to prefix your document with needs to be part of your editor schema. Ideally, any React PDF-compatible markup would be supported.

Additional context

Happy to submit a PR but first I wanted to verify this matches the intent of the PDF export package.

Bonus

• I'm a sponsor and would appreciate if you could look into this sooner than later 💖

(https://github.com/poggiolabs is a sponsor)

[YousefED]

Hi!

Thanks for the feedback. A currently possible alternative is to subclass PDFExporter and override transformBlocks to first return a prefix <View> and then call super.transformBlocks().

Let me know how this goes! I agree this might not be very clear, so happy to accept a PR with a clearer API and / or documentation.