[docs] reorganize 2.0.0 feature documentation structure

[v-kkhuang]

What is the purpose of the change

Background/Problem:
Currently, feature documentation for version 2.0.0 is scattered across multiple dev-1.19.0-* directories, making it difficult to manage and maintain. The fragmented structure reduces documentation discoverability and maintenance efficiency.

Purpose of Change:
To address this problem, this PR consolidates all design documents, requirement documents, and test documents from various dev-1.19.0-* directories into a unified docs/dev-2.0.0/ directory structure with proper subdirectories for design, requirements, features, and testing.

Value/Impact:
After the change, all 2.0.0 related documentation is organized in one centralized location, improving documentation discoverability, maintenance efficiency, and providing clear version separation.

Related issues/PRs

Related issues: close #993
Related pr:none

Brief change log

• Move design documents from dev-1.19.0-* to dev-2.0.0/design/
• Move requirement documents from dev-1.19.0-* to dev-2.0.0/requirements/
• Move feature files from dev-1.19.0-* to dev-2.0.0/features/
• Move test documents from dev-1.19.0-* to dev-2.0.0/testing/
• Remove obsolete test reports and temporary files
• Update .gitignore configuration

Checklist

• I have read the Contributing Guidelines on pull requests.
• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible
• If this is a code change: I have written unit tests to fully verify the new behavior.

[casionone]

LGTM.