update with new mdbook index.hbs to fix sidebar

[gabydd]

fixes the sidebar problems after we updated to the latest release of mdbook cc: @zqianem who was the last one to work on this part of the book

[the-mikedavis]

Odd, I see the odd sidebar behavior on https://docs.helix-editor.com/master/ but not when running locally 🤔

(Upstream template: https://github.com/rust-lang/mdBook/blob/master/src/theme/index.hbs)

[gabydd]

Hmm I was able to see it locally maybe you have an older version of mdbook locally I tested with the binary from the latest release. In terms of the upstream template this is just the upstream template with the colibri theme added as one of the theme options, which I believe was the only change for the older template as well

[zqianem]

The changes look good to me.

It might be a good idea to pin the mdBook version and document that index.hbs needs to be merged from upstream when updating the pin, to avoid breakages like this in the future. We would be able to unpin after rust-lang/mdBook#2059 allows us to avoid overriding index.hbs.