Treat `index.md` as the group index

Summary

At the moment (when !29 (merged) lands) index.md is treated as another note which both looks a bit weird and means groups cannot have a customisable intro or heading page.

Proposal

• Treat the index.md file as you would expect a index.html file - load it when you navigate to a groups root and load it when you click on the group from the sidebar. When creating the tree from the gitlab API load the index.md file in a separate field index: alongside the current tree: field.