Additional build system documentation?

[zmitchell]

Hi, I'm digging into the build system and wanted to see if you would be accepting of additional documentation. My thinking is that the current build system documentation walks you through how to write certain build programs, but doesn't explain the entities involved e.g. "executable", "module", "step", "install artifact", etc.

So, what I was thinking of contributing is a small section that explains those differences so that readers know which b.* method to call and when. As a new user myself, and because my current understanding is lacking, my current strategy is to guide myself by compiler/build system errors until something works properly. I'm hoping that we can provide a mental model that users can internalize.

[zmitchell]

I added a draft PR with a first stab at this.