Document existing generic evaluators

Problem to Solve

The CEF repository contains a set of generic evaluators in cef/core/evaluators/prebuilt that can be used in different pipelines. While the repository already includes general information about building different types of evaluators in doc/evaluators, it lacks documentation for the already pre-implemented generic evaluators.

Proposal

Document the evaluators located in cef/core/evaluators/prebuilt. Put the docs in doc/evaluators/prebuilt.