docs: Update docs of Rollout spec to add active/previewMetadata

[kzap]

Add blueGreen activeMetadata / previewMetadata as its part of the spec.

This was added back in #974 , its part of the docs in https://github.com/argoproj/argo-rollouts/blob/master/docs/features/ephemeral-metadata.md but not documented as part of the spec

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore. - chore
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[sonarcloud]

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[github-actions]

Go Published Test Results

1 988 tests   1 988 ✔️  2m 35s ⏱️
   118 suites         0 💤
       1 files           0 ❌

Results for commit 355b9d4.

[github-actions]

E2E Tests Published Test Results

    4 files      4 suites   3h 22m 58s ⏱️
  96 tests   87 ✔️   5 💤   4 ❌
394 runs  364 ✔️ 20 💤 10 ❌

For more details on these failures, see this check.

Results for commit 355b9d4.