update docs/stacker_yaml.md

[mikemccracken]

What type of PR is this?

documentation

Which issue does this PR fix:

• reorganize stacker_yaml.md slightly for better readability

• give more examples of valid urls for type 'tar', based on what the code supports.

• clarify that the type: oci does not require that the url starts with oci:, in fact that will make it fail unless your oci layout is named oci, because in pkg/types/imagesource.go ContainersImageURL() prepends oci: to the url field of type: oci imagesource structs.

What does this PR do / Why do we need it:

updates docs

If an issue # is not available please add repro steps and logs showing the issue:

Testing done on this change:

you only live once

Automation added to e2e: no

Will this break upgrades or downgrades?

no

Does this PR introduce any user-facing change?:

no

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.67%. Comparing base (46064c7) to head (0a37057). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #650 +/- ## ========================================== + Coverage 54.00% 63.67% +9.66% ========================================== Files 57 57 Lines 6175 5018 -1157 ========================================== - Hits 3335 3195 -140 + Misses 2194 1177 -1017 Partials 646 646 ```

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