📝 Create a Developer Onboarding Guide (user_guide.md)

[suleymansurucu]

As the original creator of the Sara Baby app, I’d like to share my experience with new contributors and help them get started smoothly.
This task is about writing a welcoming and practical onboarding guide for developers who clone this repository and want to contribute.

The guide should reflect the lessons learned while building the app and help others:

• Understand the project’s purpose
• Set up the environment
• Explore the folder structure
• Run the app locally
• Find where to contribute

Tasks

• Create a new Markdown file at docs/user_guide.md
• Include a warm welcome message from the creator (see example below)
• Write clear steps for:
  • Flutter & Firebase setup
  • Running the app locally
  • Understanding the project structure
  • Opening issues / submitting PRs
• Link this file in README.md under a section like:

## 📘 Developer Onboarding Guide
If you're new to the project, check out the [Developer Guide](docs/user_guide.md)