Improve exports of spec/RFCs and other documentation parts on the website

[lubianat]

Quick user story:

I am taking a long plane and I'd like to read different parts of the documentation on an eReader, offline. For that, I'd like to be able to download PDFs and/or EPUBs of particular parts of the NGFF documentation.

For example, on https://omero.readthedocs.io it is possible to download the full docs as an EPUB:

Probably needs some thinking on the equivalent here.

RO-Crate offers a list of download formats that I find very interesting: https://www.researchobject.org/ro-crate/specification/1.2/index.html