feat: Improve docs for us and AI as well as pixi config

[lsetiawan]

This pull request significantly improves the documentation and configuration for onboarding and development workflows in the project template. The changes clarify how to use Pixi for environment management, add detailed onboarding and contribution instructions, and restructure the pixi.toml file for better modularity and maintainability.

Documentation Improvements:

• Added a comprehensive AGENTS.md file that provides detailed, step-by-step instructions for using Pixi, running pre-commit checks, onboarding, adding dependencies, and troubleshooting common issues. This serves as a guide for both AI assistants and human contributors.
• Expanded the README.md with clear sections on prerequisites, installation, onboarding, project structure, available environments, development workflows, and contribution guidelines, making it easier for new users to get started and understand project conventions.

Configuration and Environment Management:

• Refactored and modularized pixi.toml:
  • Introduced [environments] for defining default and onboard environments with associated features.
  • Defined feature-specific dependencies and tasks under [feature.<name>] sections, improving clarity and maintainability.
  • Added descriptions to tasks and clarified dependency sources (Conda vs PyPI).
  • Ensured onboarding tools like ssec-cli are only included in the appropriate environment.

These changes collectively provide a much smoother onboarding experience, enforce best practices, and make the project template more robust and user-friendly for both new and experienced contributors.

[nikiburggraf]

LGTM!