Site-wide ToC v.3

[olayway]

Follow up to #94

Acceptance

• [ ] generate sitemap json and use it across the site instead of generating the sitemap on the fly, for each page that has showSidebar set to true
• [ ] mobile support
• [ ] include/exclude content dir subfolders/files
• [ ] prettier design
  • e.g. like here https://nextra.site/docs/docs-theme/page-configuration

Maybe

• [ ] allow for adding secondary navigation links
  • e.g. like here https://tailwindcss.com/docs/installation