Add --output-dir argument for custom JSON output path

[A-KumarSharma]

Changes

• Added new --output-dir CLI argument
• Users can now customize where generated JSON files are saved
• Replaced hardcoded ./results directory with configurable output path
• Added automatic directory creation using os.makedirs(..., exist_ok=True)

Example

Before:
python main.py --pdf_path sample.pdf

Output:
./results/sample_structure.json

Now:
python main.py --pdf_path sample.pdf --output-dir ./my_output

Output:
./my_output/sample_structure.json