Fix #1820: Automate directory structure documentation via script and Makefile integration

[kutumia]

This PR addresses Issue #1820 by automating the generation of PEcAn's directory structure documentation.

1. Changes Made: Added a new script: scripts/generate_dir_structure.sh

2. Recursively lists all folders in the project Outputs results to documentation/directory_structure.md in a clean Markdown format

3. Updated scripts/README.md Included description of the new script under Documentation and Dependencies

4. Updated the root Makefile Introduced a new target: generate-dir-doc This target runs the script automatically as part of the documentation workflow

How to Use: To regenerate the directory documentation manually, run in bash: make generate-dir-doc

This will update documentation/directory_structure.md with the latest folder structure.