Rework examples for predefined rules

[hudlow]

From the updated README.md:

The nested additional-examples directory has additional examples with changes represented in a sparse directory tree which can, in turn, have its own additional-examples directory. Running make ci will perform validations on the current directory, and recursively copy itself and the rest of the boilerplate into the tree of additional-examples directories to construct and then run validations for each override scenario.

[jrinehart-buf]

1. I've fixed CI.
2. @hudlow and I discussed renaming "supplementary" to support better discoverability and he's working on the change.

[jrinehart-buf]

Approving. Adding @timostamm as a reviewer for the .proto: let's make sure we're fixing the initial issue with predefined rules examples.

[timostamm]

Left one comment, but don't want to block merge. This is great.