Document user-facing `.buckconfig` options and project/cell relationships

A usable `.buckconfig` file is simple enough, but a more thorough one can be [surprisingly long-winded and non-obvious](https://github.com/thoughtpolice/buck2-nix/blob/main/.buckconfig); `target_platform_detector_spec` is the biggest offender of that fact I think (and I only kind of *think* I know what it's saying based on a pretty good guess)

It would be good to have up-to-date docs on these options. Also, it would be nice to clarify the relationship between projects/cells and buckconfig files e.g. the need for .buckconfig in sub-cells, what properties are inherited, etc.

IMO the best way to do this is to generate these directly from the source code somehow, but in practice that's potentially tricky so just writing them out by hand is good too.