Add TH helpers for stable documentation references

mgsloan [commented in #1723](https://github.com/commercialhaskell/stack/issues/1723#issuecomment-178373108):

> Perhaps this is getting a little too fancy, but it'd be nice to have a way to get strings that stabley reference the docs, in general. I'm imagining something like `$(docSection "build_command", "target-syntax")`, which would check if the build_command.md file exists, and that the anchor "target-syntax" exists.

And:

> Probably only worth it if we make a concerted effort to link to the docs a lot from CLI output, which could be a rather good thing to do.