Align Search package skill with generated skill guidance

[hizixin]

Description

Follow-up to #46710. This updates the azure-search-documents package skill to keep the generated skill structure while restoring Search-specific guidance from the previous skill.

This also resolves the review comments from #46710.

Key changes:

• Add release guidance for preview vs GA branches, branch-dependent API surfaces, and Search-specific CHANGELOG rules.
• Add testing guidance for unit tests, playback, live recording, and test authoring.
• Add a package venv helper and safe live-test environment sample.
• Clarify customization boundaries for generated code and _patch.py.
• Move release-specific guidance from architecture.md into focused release.md and testing.md references.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.