sidebar: better support index-like sources

[jorgeorpinel]

Removing source from things might break the "Edit on GitHub" links unless we update the sidebar logic to better accept them. Might not, but we need to make sure to check.

Originally posted by @rogermparent in treeverse/dvc.org#3941 (comment)

[yathomasi]

https://github.com/iterative/dvc.org/pull/3941/files#r970596286