Update imgpkg docs with debug example

[seemiller]

imgpkg Issue 130 adds a debug flag to output detailed ggcr logs. The issue also stated to add documentation.

This PR updates the imgpkg documentation to show an example usage of the debug option.

[DennisDenuto]

@seemiller

Thanks for adding documentation around the debug flag!

Step 4 is talking about kbld and deploying onto k8s. So having an imgpkg debug as step 5 seems a little off (the previous step didn't require any imgpkg interaction).

What do you think about moving the debug documentation to the FAQ section? As a 'debugging' section (under security).

We could then reference that debugging section from the basic workflow documentation if we wanted.

[seemiller]

@DennisDenuto I've updated the docs based on your suggestion

[DennisDenuto]

Thanks @seemiller LGTM