docs: add contributor ladder

[yanmxa]

Add a CONTRIBUTOR_LADDER.md — the project had an OWNERS file with reviewers/approvers but nothing documenting what those roles do or how contributors advance between them.

• Defines four rungs — Contributor → Reviewer → Approver → Maintainer — mapped directly onto the OWNERS file.
• Follows the CNCF contributor-ladder conventions (Responsibilities/Requirements/Privileges structure, per-role promotion process, and the standard Inactivity / Involuntary Removal / Emeritus / Contact sections).
• Links the ladder from CONTRIBUTING.md.

[san-ci]

[REVIEW NOTIFIER] This PR requires a review from @zhujian7.

Once this PR has been reviewed and has the lgtm label, it will be automatically squash-merged.

Suggested reviewer

Based on OWNERS, @zhujian7 is suggested to review this PR.

How to get your PR reviewed:

• Address reviewer feedback and push changes
• Reviewer adds lgtm label when satisfied
• An approving review + lgtm will trigger auto-merge

[yanmxa]

cc @ldpliu @zhujian7 @hchenxa — this just writes down the roles our OWNERS file already implies (Contributor → Reviewer → Approver → Maintainer). The thresholds (e.g. ~3 months to Approver, 6-month inactivity) are starting points — please push back on anything that doesn't fit before we merge.

[hchenxa]

@yanmxa it looks good

[san-ci]

[REBASE NOTIFIER] Hi @yanmxa! This PR is behind the base branch and needs to be rebased.

The lgtm label has been removed and the reviewer will need to re-review after you fix this.

How to fix this

rebase your branch — Run git rebase main locally, then force-push the updated branch.

After fixing:

• The need-rebase label will be automatically removed
• Request a new review from the OWNERS file
• The reviewer can add lgtm once satisfied

[san-ci]

[REBASE NOTIFIER] Hi @yanmxa! This PR is behind the base branch and needs to be rebased.

The lgtm label has been removed and the reviewer will need to re-review after you fix this.

How to fix this

rebase your branch — Run git rebase main locally, then force-push the updated branch.

After fixing:

• The need-rebase label will be automatically removed
• Request a new review from the OWNERS file
• The reviewer can add lgtm once satisfied

[san-ci]

[REBASE NOTIFIER] Hi @yanmxa! This PR is behind the base branch and needs to be rebased.

The lgtm label has been removed and the reviewer will need to re-review after you fix this.

How to fix this

rebase your branch — Run git rebase main locally, then force-push the updated branch.

After fixing:

• The need-rebase label will be automatically removed
• Request a new review from the OWNERS file
• The reviewer can add lgtm once satisfied

[san-ci]

@yanmxa cannot add the lgtm label to their own PR. Another approver must review this PR and add the label.

[san-ci]

[APPROVALNOTIFIER] This PR has the lgtm label but is NOT APPROVED yet.

Please request an approving review from @ldpliu (see OWNERS).

Once approved, the Review Gate will automatically squash-merge this PR.