Add community forks documentation page and mkdocs link

[Bhumika72248]

This PR adds a new documentation page (docs/community-forks.md) that lists community-maintained forks of cookiecutter-data-science for more specific purposes.

Added links:

• Deep Learning fork: https://github.com/tdeboissiere/cookiecutter-deeplearning
• Snakemake pipeline fork: https://github.com/xguse/cookiecutter-snakemake-analysis-pipeline

Also updated mkdocs.yml to include this page in the documentation navigation.