Update sidebar component (top left)

- Make sure all level 1 index pages (Foundations, Experience Patterns, Delivery Manual, Resources, Governance) sit at the top of the sidebar
- Make it more clear that the level 1 index pages are above the sub pages in the information architecture (e.g. by bolding them and sorting the sub-pages underneath)
- Bonus: if possible, allow the sidebar to show the entire page tree, not just that specific section (similar to https://uncefact.github.io/project-crm/docs/about )