Add NFDB cleaning notebook, updated README, and .gitignore (Milestone 2)

[Vahablotfi]

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This PR adds the Milestone 2 data preparation work.

Changes

• Added 2_data_preparation/clean_nfdb_all.ipynb – Jupyter notebook to clean NFDB point data
• Updated 2_data_preparation/README.md – detailed cleaning process and steps
• Updated .gitignore – excludes large data files (CSV, shapefiles)

General Checks

• the branch is up to date with main
• the code works when pulled and run locally
• All CI checks pass (or at least discussed)
• all conflicts are resolved (if any)
• PR has a descriptive title
• PR has appropriate labels and milestones for easy identification
• PR it is assigned to the owner
• reviewers are assigned
• the PR contributes only one focused change
• It is in the appropriate column in the project board (if necessary)
• has short and clear description
• is linked to an issue (if it is related)
• feedback is addressed (if any and if it is appropriate feedback.)

Markdown

• the markdown source is formatted
• spelling and grammar is correct in all text
• The markdown looks correct when you preview the file
• all links and images work