contents list in docs

Stream: general

[Bhavik Mehta]

Many mathlib files have sections, which are helpful for organisation. In Haskell documentation, these sections show up in a contents bar on the left, eg here: https://hackage.haskell.org/package/base-4.16.3.0/docs/Data-Monoid.html. I would find this really helpful for navigating the API within a file. Can we have something similar for doc-gen?