Improve skaffold help output

[medyagh]

Matches kubectl help style, categorizes skaffold help output to categorize under:

• Pipeline building blocks for CI/CD
• Getting started with a new project
• Utilities

Closes this #1580

[priyawadhwa]

Just some docs errors!

--- FAIL: TestPrintMan (0.06s)
    man_test.go:52: You have skaffold command changes but haven't generated the CLI reference docs. Please run ./hack/generate-man.sh and commit the results!

[container-tools-bot]

Please visit http://35.235.114.123:1313 to view changes to the docs.

[dgageot]

@medyagh there's a test (TestMainHelp) that verifies a few words in the skaffold help output.

[balopat]

hehe, we missed that (I was pairing on this with Medya), thanks for the pointer :)

[dgageot]

ping @medyagh do you need any help to fix this PR?

[medyagh]

sorry for the delay, I am on it now

[codecov]

Codecov Report

Merging #2324 into master will increase coverage by 5.5%.
The diff coverage is 77.77%.

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/cmd.go	69.36% <77.77%> (+4.66%)	⬆️
pkg/skaffold/docker/image_util.go	0% <0%> (-33.34%)	⬇️
pkg/skaffold/docker/remote.go	35.41% <0%> (-27.09%)	⬇️
pkg/skaffold/build/bazel/dependencies.go	80.76% <0%> (-12.57%)	⬇️
pkg/skaffold/util/port.go	85.18% <0%> (-7.41%)	⬇️
pkg/skaffold/build/cluster/logs.go	15.15% <0%> (-4.85%)	⬇️
cmd/skaffold/app/cmd/debug.go	50% <0%> (-4.55%)	⬇️
pkg/skaffold/schema/v1beta9/upgrade.go	85.71% <0%> (-3.33%)	⬇️
pkg/skaffold/schema/profiles.go	86.2% <0%> (-1.48%)	⬇️
pkg/skaffold/runner/build_deploy.go	51.35% <0%> (-1.43%)	⬇️
... and 53 more