Improve animint2pages by automatically taking screenshot of the animint

[siddhesh195]

As a part of GSoC 2024 one of my task is to improve animint2pages function by automatically taking screenshot of the animint. Automatic screenshot will automate the manual step of taking a screenshot for the Github pages
gallery.

[tdhock]

related to #115 -- often you want a screenshot of the viz after the ggplots have been arranged in a certain way on the page -- with current code this is done interactively (resize % zoom / window height/width) but would be nice to have the ggplot layout defined in code and then guaranteed at render time (no need for interactive resizing)

[tdhock]

crop white space https://imagemagick.org/Usage/crop/
R bindings https://cran.r-project.org/package=magick