Rearranging documentations in the sidebar / results

I'd like to be able to rearrange documentations in the sidebar by dragging them in the "Select documentation" list. The order should also be reflected in the search (when results have identical scores).

Start by tracing the sidebar's “Select documentation” list and the search result ordering logic. The work is done when documentation entries can be reordered by dragging and equal-scored search results follow that order; no files or tests are named in the issue.

索引モデルが issue の本文から書いたものです。