add build graph documentation

[zmitchell]

This adds a description of the build graph and how it's made up of Steps. Previously it was confusing whether there were other named entities ("artifact", "executable", etc) that a user needed to know about, but hopefully this clarifies that methods like addExecutable and installArtifact are doing similar things under the hood (e.g. creating Steps).

Addresses https://github.com/ziglang/www.ziglang.org/issues/561

[alexrp]

What's with the Flox stuff?

[zmitchell]

Ah, that was my development environment. Didn’t mean to include that.

[zmitchell]

I've removed the commit with my development environment. Any feedback on the content?