docs: clarify OCI and Docker exporter output modes

[codewithdaniel1]

Description

Clarifies the dest and tar parameter behavior for the OCI and Docker exporters.

This PR updates content/manuals/build/exporters/oci-docker.md to:

• replace the terse dest description with a clearer destination-path description
• clarify that tar bundles the output into a tar archive
• add an Output modes section with examples for:
  • exporting an OCI image layout as a tar archive
  • exporting an OCI image layout as an unpacked directory with tar=false
  • using the Docker exporter without dest to load the image to the local image store
  • using the Docker exporter with dest to save the image layout to disk

Related issues or tickets

Closes #25663

Reviews

• Technical review
• Editorial review
• Product review

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