doc: require a paired hugegraph-doc PR for feature and config changes

[bitflicker64]

Purpose of the PR

Draft for discussion, companion to #3197.

The PR template already has a Documentation Status section, so the intent is there. The
problem is the first option:

- [ ] `Doc - TODO`  <!-- Your PR changes impact docs and you will update later -->

"You will update later" is a promise with nothing attached to it. No link, no issue, no
follow-up, so once the code PR merges the box is the only record that documentation was
owed, and it stops being visible to anyone. The result is documentation that describes an
older version of the code, which readers and search tools then pick up as current.

CONTRIBUTING.md does not mention documentation at all in the section on opening a PR,
which is the point where a contributor decides whether to write any.

Main Changes

• .github/PULL_REQUEST_TEMPLATE.md: Doc - TODO now asks for a link, either the
  hugegraph-doc PR or a tracking issue. Same three options, same structure, one comment
  reworded.
• docs/CONTRIBUTING.md: one paragraph in section 4, saying that a change to a feature, a
  configuration item or a Dockerfile opens the matching hugegraph-doc PR at the same
  time, and that the two merge together.

7 lines added, 1 reworded.

Two open points for review:

1. Should Doc - TODO require a link, or be removed entirely so the choice is Done or
   No Need? Requiring a link is the softer option and is what this PR does.
2. CONTRIBUTING.md says the website copy at
   https://hugegraph.apache.org/docs/contribution-guidelines/ is authoritative. If this
   lands, the same paragraph needs a matching hugegraph-doc PR, which is the rule applying
   to itself.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • xxx

Does this PR potentially affect the following parts?

• Dependencies (add/update license info & regenerate_known_dependencies.sh)
• Modify configurations
• The public API
• Other affects (typed here)
  • Changes the PR template every contributor sees, and the contribution guide.
• Nope

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[bitflicker64]

Folded into #3197, which now carries the AGENTS.md, PR template and CONTRIBUTING changes as one change. Nothing here is lost.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.79%. Comparing base (e85b36a) to head (74c31fb).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3198   +/-   ##
=========================================
  Coverage     37.79%   37.79%           
- Complexity     6563     6566    +3     
=========================================
  Files           800      800           
  Lines         68960    68960           
  Branches       9166     9166           
=========================================
+ Hits          26061    26066    +5     
+ Misses        39834    39829    -5     
  Partials       3065     3065           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.