Add help docs for makefile

[hi-rustin]

Is your feature request related to a problem?

For now, we have a bunch of make commands but no docs. It isn't easy to find the available commands.

Describe the feature you'd like

Adapt from https://www.thapaliya.com/en/writings/well-documented-makefiles/ and add help docs for each command.

Describe alternatives you've considered

No response

Teachability, Documentation, Adoption, Migration Strategy

No response

[hi-rustin]

Introduced in https://github.com/pingcap/tiflow/pull/6568

We need to add help for other targets.