Test and document gdiff workflow

[hturner]

With #230, we have at least some of the prerequisites for visual testing with {gdiff}. However, we should try out the workflow in the updated container and see if anything else is missing.

If everything works, we should document it!

I wrote some notes on running gdiff testing on macOS, where I hit several issues with building packages from source: https://github.com/r-devel/rcwg/blob/main/working_documents/gdiff_notes.md. I think it should be easier in the container as we can install linux binaries (see the footnotes in the working document). We can also provide the example R scripts for running the tests, to make it as straight-forward as possible.

This would be a new chapter after the multiple build chapter.

[hturner]

I just tried installing {gdiff} in the container and we need one more prerequisite: libpoppler-cpp-dev, so this needs to be installed alongside libmagick++-dev(add it on the same line in the Dockerfile, with a comment above the code block to explain why we are adding these two packages).

[hturner]

Regarding the user using linux binaries (when they want to install packages in either R-release or their builds of R-devel), we should use the .Rprofile approach described in https://docs.r-universe.dev/install/binaries.html#how-to-install-linux-binary-packages, except we should edit the .Rprofile in the root of the workspace rather than creating a ~/.Rprofile (since we want the .Rprofile to be visible to the user).

[hturner]

Removing v0.4 milestone - we think we have the prerequisites now, testing and documenting can be delayed to next release.