Improve rake --tasks

[ybiquitous]

This PR makes rake --tasks output useful information.

$ rake --tasks
rake debug:latest_full_version    # Check if the specified Ruby version is the latest full version (env: ruby_version)
rake debug:latest_stable_version  # Show the latest stable Ruby version
rake debug:latest_version         # Check if the specified Ruby version is the latest (env: ruby_version)
rake debug:version_exist          # Check if the specified Ruby version exists (env: ruby_version)
rake debug:versions               # Display all available Ruby versions grouped by major.minor
rake docker:build                 # Build Docker image for Ruby (env: ruby_version, ubuntu_version, arch, image_version_suffix, tag_suffix, tag, cppflags, optflags, target)
rake docker:manifest:create       # Create multi-architecture Docker manifests (env: ruby_version, architectures, manifest_suffix, image_version_suffix)
rake docker:manifest:push         # Push multi-architecture Docker manifests to registry (env: ruby_version, image_version_suffix)

[hsbt]

👍