`conda-build` is not referenced at all in the docs

I just realized that on the Conda-Forge documentation we do not refer to [`conda-build`'s documentation](https://docs.conda.io/projects/conda-build/en/latest/) at all, which is very surprising. I am not sure if this is done on purpose, but when I started (and even after becoming an experienced feedstock maintainer) I had to refer to it more than often, because it is one of the pillars of Conda-Forge's infrastructure.

Over the time I found the following pages are the most useful ones:
- https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html
- https://docs.conda.io/projects/conda-build/en/latest/resources/activate-scripts.html
- https://docs.conda.io/projects/conda-build/en/latest/resources/link-scripts.html
- https://docs.conda.io/projects/conda-build/en/latest/user-guide/environment-variables.html

To the minimal extent, we should at least mention that this documentation exists so that advanced users/maintainers know where to look. Depending on the work scope, one may further consider incorporating some of the materials from there.