docs(contributing): document issue assignment commands

[jackeyfaker77]

Summary

Clarify and document issue claiming commands (.take / /take) in the contributing documentation.

Fixes #4043

Verification

• Reviewed the updated documentation for clarity and accurate formatting.
• Checked rendered markdown preview locally.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

The added copy matches the handler that already landed on main in #3774 (.github/workflows/take.yml). That workflow is the only issue-comment assigner on main today; there is no .take, /take, .untake, or /untake matcher.

EN and ZH both document an entire comment body of exactly take / untake. That is what take.yml listens for (no trim, issues only). #4043 is addressed.

Non-blocking: please edit the GitHub Summary from .take / /take to take / untake so the thread does not teach strings that do not work.