docs: fix documentation drift and add docs validation

[TATP-233]

Summary

This PR fixes issue #162 by addressing documentation drift and adding a docs validation script.

Changes

1. Fixed outdated CLI commands in inhand_rot_allegro/README.md

• Updated from argparse style (--task, --env_num, etc.) to Hydra style (task=, algo.num_envs=)
• Changed python to uv run python to follow project conventions
• Fixed path to gen_grasp.py (added src/ prefix)
• Added note about argparse vs Hydra usage distinction

2. Fixed non-existent language directory references

• Removed broken links to ../en/, ../ja/, ../ko/ directories in all zh_CN docs
• These directories do not exist in the repository

3. Added docs validation script (scripts/check_docs.py)

Validates documentation for:

• Script references (scripts/*.py must exist)
• Hydra config keys (common keys must match expected patterns)
• Relative links (markdown links must point to existing files)
• Outdated argparse flags (flags like --task that should be Hydra style)

Validation

• make check passes
• make test passes
• Docs validation script runs successfully

Definition of Done

• README, CONTRIBUTING, env README and training docs command styles are consistent
• No incorrect script paths or deprecated parameters in active docs
• Docs-only changes can trigger minimal docs validation
• Documentation structure rules match current repository layout

Fixes #162