Add hints for debugging to the docs

[DavHau]

trafficstars

eg: use nix repl to explore .config attribute etc.

[eflanagan0]

Where is the source code for the documentation? I have started debugging using nix repl> :lf outputs and can add that tip.

[DavHau]

It's currently at modules/flake-parts/website/site

Maybe we should move it somewhere where it's easier to discover.