add meta descriptions plugin
draft status; this is a breaking change which will require installation of mkdocs-meta-descriptions-plugin
locally.
installs https://pypi.org/project/mkdocs-meta-descriptions-plugin/ to generate meta descriptions for each docs page based on the first paragraph of each page as part of the build; right now each page's meta description is just something to the effect of "NERSC documentation" which is not doing us or users any favors in the SEO context.