Type-based search in the docs

[lf-]

Problem:

I want to find where to get a ProjectRoot from, for example. Currently, the only way to find this is to grep the buck2 sources or remember how to build the docs markdown and then grep the docs markdown.

Solution:

Build an API search that has a structured representation of the buck2 APIs and maybe even the prelude functions. I mean I guess this is Glean in a sense, but https://github.com/facebookincubator/Glean/issues/594