docs: add TrainJob to Job-integrated features table on Overview

[NarayanaSabari]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Add the TrainJob column to the “Job-integrated features” table on the Overview page and mark the features implemented by TrainJob with +. This update keeps documentation synchronized with the current TrainJob integration implemented in the codebase.

Which issue(s) this PR fixes:

Fixes #7864

Special notes for your reviewer:

• This is a documentation-only change: site/content/en/docs/overview/_index.md.
• Verified that TrainJob integration is implemented and covered by tests; references:
  • Partial admission / RunWithPodSetsInfo: pkg/controller/jobs/trainjob/trainjob_controller.go#L245-286
  • All-or-nothing (PodsReady): pkg/controller/jobs/trainjob/trainjob_controller.go#L335-350
  • Dynamic reclaim (ReclaimablePods): pkg/controller/jobs/trainjob/trainjob_controller.go#L352-376
  • MultiKueue integration adapters: pkg/controller/jobs/trainjob/trainjob_multikueue_adapter.go#L44-88, #L104-115
  • Webhook defaulting & validation (TAS): pkg/controller/jobs/trainjob/trainjob_webhook.go#L69-90, #L145-160
  • Manager config includes trainer.kubeflow.org/trainjob: config/components/manager/controller_manager_config.yaml#L56-60
  • Test coverage examples:
    • test/e2e/singlecluster/trainjob_test.go
    • test/integration/singlecluster/controller/jobs/trainjob/trainjob_controller_test.go
    • test/e2e/tas/trainjob_test.go
• I did not update any localized docs per request (no zh-CN changes).
• This PR is intended to update the Overview table only and does not make code changes.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @NarayanaSabari. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mimowo]

/ok-to-test

[mimowo]

This feature gate is not relevant for the TrainJob. Actually we can remove the row, it is only relavant for batch/Job

[mimowo]

LGTM, just two nits

[tenzen-y]

/release-note-none