Update mkdocs theme and template variables.

[AnnaSyme]

error message (e.g. after "mkdocs serve"):

WARNING - Your theme does not appear to contain a 'main.html' template. The 'base.html' template was used instead, which is deprecated. Update your theme so that the primary entry point is 'main.html'.

WARNING - Template variable warning: 'page_description' is being deprecated and will not be available in a future version. Use 'config.site_description' instead.

(and other template variable warnings).