Improved documentation

I'm starting this issue just to brainstorm some ideas for improving the package documentation.

One way to structure the documentation is to essentially move a lot of what is in the main readme file to separate articles. Perhaps these articles:

• Installation: I think a separate page on this might be nice since the chrome requirement is easy to miss. 
• Compile slides to an HTML page: This would just be build_html(). It probably doesn't need it's own page, but I don't konw where else to put it.
• Build slides to a PDF: PDF is probably the most common output type, plus the complex / partial slides bit could be explained here in detail.
• Get PNGs of all or select slides: This would include build_png() and build_social(). Examples could include building from a {xaringan} deck (the .Rmd file) as well as perhaps building from a PDF of a .pptx file.
• Animate your slides as a GIF or MP4: This would include build_gif() and build_mp4(). Again, could show examples of building from the .Rmd file or a PDF of a .pptx file.
• Convert your slides to a PPTX: This would include build_pptx().

Start by reviewing the main README and the documentation ideas listed in this issue, including the chrome requirement and the build_html(), build_png(), build_social(), build_gif(), build_mp4(), and build_pptx() functions. Decide how the proposed articles should be organized and what examples each should cover; the work is done when the documentation structure and coverage are clearly defined and implemented.