docs: add initial GPU explanation

[dsloanm]

Adds sections to Explanation for the GPU support recently merged into the Slurm charms. Once we have Reference docs, we'll likely want to update these sections to include links to at least a table of supported GPU models.

I've gone with Driver auto-install and Slurm enlistment as subsection titles but wonder if these should be GPU driver auto-install and Slurm enlistment of GPUs. This would make the context obvious when, e.g., looking through search results, however would mean a lot of repetition of "GPU". Let me know your thoughts.