Change default make target and expand make help documentation

[andyasp]

Builds off of the changes made in #86.

This changes the default goal (only running make) back to building the binary. It was changed to help before, but having to type make rollout-operator is a bit rough. I kept the help target at the top of the Makefile to keep the discoverability of the option since it is nice.

I also modified the awk command used in the help target to allow documenting the options with dependencies and then documented them. make help now prints:

Usage:
  make <target>

Targets:
  help                                           Display this help and any documented user-facing targets
  rollout-operator                               Build the rollout-operator binary
  rollout-operator-boringcrypto                  Build the rollout-operator binary with boringcrypto
  build-image                                    Build the rollout-operator image
  build-image-boringcrypto                       Build the rollout-operator image with boringcrypto
  publish-images                                 Build and publish both the standard and boringcrypto images
  publish-standard-image                         Build and publish only the standard rollout-operator image
  publish-boringcrypto-image                     Build and publish only the boring-crypto rollout-operator image
  test                                           Run tests
  test-boringcrypto                              Run tests with GOEXPERIMENT=boringcrypto
  integration                                    Run integration tests
  lint                                           Run golangci-lint
  clean                                          Run go clean and remove the rollout-operator binary