[Task] Document the split-client migration and contribution workflow

### Description

## Background

Community contributors need a clear guide for contributing to the split Java client repository and for making coordinated changes across the main ShenYu repo and `shenyu-client-java`.

## Tasks

- [ ] Explain why the Java client is split from the main repo.
- [ ] Explain the different JDK policies for client repo and main repo.
- [ ] Explain how to build and test `shenyu-client-java`.
- [ ] Explain how to run JDK 8/17/21/23/25 verification.
- [ ] Explain how to test local client artifacts with main repo admin/bootstrap.
- [ ] Explain when a change requires PRs in both repositories.
- [ ] Add a PR checklist for client repo changes.
- [ ] Add a PR checklist for main repo changes that affect client compatibility.

## Expected Output

Contributor-facing documentation.

## Acceptance Criteria

- A new contributor can understand where to make client-related changes.
- The documentation includes local verification commands.
- The documentation explains cross-repo compatibility expectations.

### Task List

_No response_

Start by locating the contributor documentation in the main ShenYu repository and the shenyu-client-java repository, then review the client build and test entry points and the admin/bootstrap integration path. Document the split rationale, JDK policies, local verification commands, cross-repository compatibility rules, and the two requested PR checklists. Done means a new contributor can identify where to change the client, verify JDK 8/17/21/23/25, and test local client artifacts.

Written by the indexing model from the issue text.