Which features should be moved to extra builds?

[fg-mindee]

As discussed in #113, there are a few troublesome dependencies of the project that may not be used by most users. So this might be the right time for release 0.6.0, to assess which features/dependencies could/should be put in an extra build.

Here is a shortlist of such features:

• PDF reading/rendering (doctr.io.pdf): we have to rely on third-party libraries, that usually have low-level bindings and usually no support for conda build for instance. There are only a few that are compatible with docTR license-wise, and we have already conducted an extensive search for all options (Consider removing PyMuPDF for dependency that is not AGPL licensed #486)
• HTML rendering as PDF (doctr.io.html): perhaps we should check alternatives, but weasyprint has generated problems for several users, especially on Windows (cf. cannot load library 'pango-1.0': error 0x7e. Additionally, ctypes.util.find_library() did not manage to locate a library called 'pango-1.0' #815)

Are there any others? What should we do about those?
cc @charlesmindee