Add 'concepts' section in the documentation.

### Where should the content be added?

Not sure where. Maybe after the introduction.

### What should be added?

A newbie-- like me-- may not be aware of the terms 'recipe', 'feedstock', etc.

A section, namely 'concepts', that talks about these terms in a simple way without jargon may help newcomers understand things in a better way.

### Additional information

I am not aware of the target audience for the documentation. So this may be an irrelevant or a helpful issue.