docs: sync release version to 6.0.0 and add docs version consistency check

[SatoryKono]

Motivation

• Ensure documentation Current Version is accurate for the v6.0.0 release and prevent future desynchronization between pyproject.toml and docs.
• Make the version check part of contributor workflow so releases always include the docs metadata update.

Description

• Updated docs/00-project/index.md Current Version to v6.0.0 with release date 2026-02-18 and refreshed the Last updated date to 2026-02-24.
• Updated docs/00-project/00-map.md header metadata to include the current release and updated last-updated date.
• Added scripts/check_docs_version_sync.py, a small validator that reads project.version from pyproject.toml and compares it to the **vX.Y.Z** (YYYY-MM-DD) entry in docs/00-project/index.md, exiting non‑zero on mismatch.
• Added a mandatory rule to .github/CONTRIBUTING.md requiring contributors to update docs/00-project/index.md for releases and to run the new check as part of release PRs.

Testing

• Ran uv run python scripts/check_docs_version_sync.py and it returned success (prints "Version sync OK"), confirming parity with pyproject.toml.
• Ran uv run ruff check scripts/check_docs_version_sync.py which passed after fixing linter-reported print usage to sys.stdout/sys.stderr writes.
• Ran uv run ruff format scripts/check_docs_version_sync.py to apply formatting; formatting step completed successfully.
• Note: an initial pre-commit run flagged unrelated repo-wide hooks (missing pip-audit executable and an existing legacy config path warning); the script was adjusted for lint rules and verification commands above were re-run successfully.

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[SatoryKono]

Superseded by #2374.