dev install Git commands

[jpn--]

This pull request adds detailed instructions for developers on how to clone and manage branches when working with the ActivitySim repository. The new documentation clarifies best practices for working with forks, reducing clone size, and fetching branches, making it easier for contributors to set up their development environment efficiently.

Documentation improvements for developer setup

• Added a recommendation to use uv sync --locked to ensure all dependencies are loaded correctly when installing ActivitySim for development.

New section on cloning and managing branches

• Introduced a comprehensive "Cloning from GitHub" section that explains how to clone the repository, encourages working on forks, and provides instructions for using the GitHub CLI.
• Provided guidance on reducing clone size and disk usage by using the --shallow-since option when cloning, helping developers avoid downloading unnecessary history and data files.
• Added step-by-step instructions for fetching specific branches and for configuring Git to fetch all branches in a fork, supporting more advanced workflows.