Add more CLI usage examples to CLI help info

[FeynmanZhou]

It's recommended to add more CLI examples to CLI help info, make sure each command and flag has corresponding examples. This will be friendly to both LLMs and human users. The format is similar to https://github.com/oras-project/oras/blob/main/cmd/oras/root/push.go#L67

Example - Push file "hi.txt" with media type "application/vnd.oci.image.layer.v1.tar" (default):
  oras push localhost:5000/hello:v1 hi.txt

Example - Push file "hi.txt" and export the pushed manifest to a specified path:
  oras push --export-manifest manifest.json localhost:5000/hello:v1 hi.txt

Example - Push file "hi.txt" with the custom media type "application/vnd.me.hi":
  oras push localhost:5000/hello:v1 hi.txt:application/vnd.me.hi

Example - Push multiple files with different media types:
  oras push localhost:5000/hello:v1 hi.txt:application/vnd.me.hi bye.txt:application/vnd.me.bye

Example - Push file with colon in name "hi:txt" with the default media type:
  oras push localhost:5000/hello:v1 hi:txt:

Example - Push file "hi.txt" with artifact type "application/vnd.example+type":
  oras push --artifact-type application/vnd.example+type localhost:5000/hello:v1 hi.txt