Update documentation and refactor type checks using `{rlang}` functions

[elipousson]

This pull request exports the documentation and clears all of the errors and notes from devtools::check() per #4. There is still the outstanding warning I mentioned in the comments for that issue.

A few things to flag when you have a chance to review:

• I removed httr2 and Rcpp11 from Imports entirely because they appeared to be unused. If this is not correct, I'm happy to restore those dependencies.
• I switched the error messages to cli_abort() because I'm a big fan of the cli formatted messages but I can switch it back to regular abort() if you'd rather.
• I renamed testfiles to pdfr_paths to make the object a bit more explicit about what it was.
• I added support for page ranges to pdfpage() but this does introduce a new bug, where it manages to crash R entirely if you try to read a page that isn't present in the PDF file. I tried to address this with rlang::try_fetch() for error handling with no luck.
• I went ahead and added myself as a contributor to the DESCRIPTION. Hope that is OK!

For what it is worth, I don't have any expectation that you'll dig back into this package if you've already put it to the side but I appreciate you considering a pull request to get the documentation into a more usable state!

[AllanCameron]

Happy to have you as a contributor Eli! Thank you. I like the suggestions. I think the multiple pdfpage bug will have to be handled at the C++ level.