docs: update oras discover JSON output format

[Iam-Karan-Suresh]

This PR updates the documentation for oras discover --format json to reflect the current output format introduced in ORAS v1.3.0.

In v1.3.0, the discover command returns the subject manifest along with its referrers, following the OCI referrers data model. The previous documentation still described the legacy manifests-based output, which is no longer returned by recent ORAS versions.

This change aligns the docs with the actual CLI behavior to avoid user confusion.

related to issue:#497

[Copilot]

Pull request overview

This PR updates the documentation for oras discover --format json to align with the new output format introduced in ORAS v1.3.0, which now returns the subject manifest along with its referrers following the OCI referrers data model, instead of the legacy manifests-based output.

Changes:

• Updated tree output example to show current format with annotations
• Updated JSON output example to reflect new structure with subject manifest at root level and referrers array instead of manifests array

Comments suppressed due to low confidence (1)

versioned_docs/version-1.3/how_to_guides/format_output.mdx:283

• The field descriptions (lines 275-283) need to be updated to match the new JSON output format. The documentation still describes the old format with a manifests array at the root level, but the actual JSON output (lines 292-310) now includes the subject manifest details at the root level with a referrers array. The description should be updated to explain that the output includes the subject manifest's metadata (reference, mediaType, digest, size) at the root level, followed by a referrers array containing the list of artifacts that reference the subject.

View an artifact's referrers manifest in pretty JSON output. The following fields should be outputted:

- `manifests`: the list of referrers
  - `reference`: full reference by digest of the referrer
  - `mediaType`: media type of the referrer
  - `size`: referrer file size in bytes
  - `digest`: digest of the referrer
  - `artifactType`: artifact type of a referrer
  - `annotations`: contains arbitrary metadata in a referrer

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