Add examples for the flag "--format"

[FeynmanZhou]

What happened in your environment?

After upgraded to ORAS v1.2.0, I noticed that there are no any examples of the flag --format in ORAS CLI command help reference. Users might be hard to understand how to use --format in ORAS CLI commands

What did you expect to happen?

It would be much user-friendly if we could add basic examples for the flag "--format" in the help reference.

How can we reproduce it?

Run "--help" in each oras command

What is the version of your ORAS CLI?

oras v1.2.0

What is your OS environment?

Ubuntu 22.06

Are you willing to submit PRs to fix it?

• Yes, I am willing to fix it.

[shizhMSFT]

We are having more and more examples in the Example section of the help. Sometimes, it is hard for me to find the usage and flag usages.

Should we consider man pages instead of --help? That is,

man oras

or maybe in PowerShell

Get-Help oras