docs: document event actors

[WenyXu]

What changed

Document the common actor column for ADMIN function and Procedure events. Explain how a Procedure keeps its actor across lifecycle events, how authentication affects the value, and what each protocol records when authentication is disabled.

Add a query example for filtering events by actor.

Closes #2735.

Scope

• Documentation versions: Nightly
• Languages: English, Chinese

Verification

• pnpm build
• DOC_LANG=zh pnpm build
• DOC_LANG=en pnpm check:links
• DOC_LANG=zh pnpm check:links
• git diff --check origin/main...HEAD
• Manually verified Procedure actor values over HTTP SQL, gRPC, MySQL, and PostgreSQL against a local cluster without authentication.
• Manually verified ADMIN event actor values over HTTP SQL, MySQL, and PostgreSQL.
• No navigation update was required because no pages or sidebar entries were added, removed, or moved.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• I updated navigation when the document structure changed.

[WenyXu]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: a873d56b8f

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[fengjiachun]

LGTM