Improve examples/ subfolder with cross-compilation and more docs

[cottand]

• Adds examples for cross-compilation of EIFs in ./examples, and gives more detail on what commands to run in order to build EIFs.
• Fixes a bug where the examples lockfile would fail to find the parent flake if the flake is not already in the Nix store.
• Updates architecture diagram to reflect changes from use AWS' aws-nitro-enclaves-image-format instead of maintaining our own CLI fork #24
• Removes quick start example as it is redundant with ./examples/ and means maintaining an extra flake in markdown