* We move all the example code from the readme to the `examples` folder.
* We provide a `scripts/build-readme.sh` script that inlines referenced
examples in `README.md.in` and outputs `README.md`. The script also
verifies that the output readme is the same as in version control.
* We provide a `scripts/build-examples.py` script that builds the Yaml
output for all examples. The script also verifies that the generated
Yaml files are the same as in version control.
As part of this I created a new files `./nixpkgs` that obtains a
specific version of the nixpkgs repo. This code was previously spread
between `release.nix` and `fetchNixpkgs.nix`. I also removed the Nix
1.11 code to simplify things. If we need Nix 1.11 support we can add it
again.
