ranking: diversity boost code if no code at top

[keegancsmith]

Another "diversity rule" that could be helpful: in the absence of file filters, at least one file in the top 3 should be a code file (not build, not docs).

Originally posted by @jtibshirani in sourcegraph/zoekt#665 (comment)

[jtibshirani]

It could be good to "step back" and take a general look at how we mix results across languages and types (source, config, docs, etc.) For example, if scores were more comparable across types, maybe we wouldn't need special diversity rules?

[jtibshirani]

Closing in favor of https://github.com/sourcegraph/sourcegraph/issues/58057