docs: mention the Kubernetes Downward API in the ephemeral metadata documentation page

[kostis-codefresh]

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.
• 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.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Published E2E Test Results

  4 files    4 suites   3h 8m 13s ⏱️
113 tests 103 ✅  7 💤 3 ❌
456 runs  424 ✅ 28 💤 4 ❌

For more details on these failures, see this check.

Results for commit 2773f3a.

[github-actions]

Published Unit Test Results

2 293 tests   2 293 ✅  2m 59s ⏱️
  128 suites      0 💤
    1 files        0 ❌

Results for commit 2773f3a.