Improve derive attribute documentation with simpler example

[nsheff]

Summary

This PR improves the derived attributes documentation by adding a simpler, beginner-friendly example at the start. This addresses user feedback in #37 that the documentation was
too complex and lacked a simple starting point.

Changes

• Added "What are derived attributes?" section with a clear, concise definition
• Added "Quick Start - Simple Example" section showing basic file path derivation
  • Uses a minimal sample table with just sample_name
  • Demonstrates simple variable substitution: /data/{sample_name}.fastq
  • Shows the concrete results of the transformation
• Renamed the existing complex example to "Advanced Example - Multiple Sources"
• Updated both the markdown file and the Jupyter notebook

Addresses

Closes #37

Before/After

Before: The documentation jumped straight into a complex example with multiple variables (organism, time) and multiple sources, which was confusing for beginners.

After: Users now see a simple 3-sample example first that clearly explains the concept, then can proceed to the more complex example once they understand the basics.

The new example demonstrates the core concept: you define a pattern in your config (e.g., my_source: /data/{sample_name}.fastq), reference it in your sample table (file_path: my_source), and peppy expands it automatically.

Base branch: mkdocstrings

[Copilot]

Pull Request Overview

This PR improves the documentation for derived attributes in peppy by adding a beginner-friendly "Quick Start" section with a simple example before diving into the more complex multi-source scenario. The changes make the documentation more accessible by providing a clear explanation of what derived attributes are and a straightforward example that users can understand immediately.

Key Changes:

• Added "What are derived attributes?" section to explain the concept
• Added "Quick Start - Simple Example" section with a minimal working example
• Renamed "Problem/Goal" section to "Advanced Example - Multiple Sources" for better clarity
• Minor formatting improvements (trailing whitespace removal)

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/peppy/notebooks/feature3_derived.ipynb	Updated notebook markdown cell with new introductory sections and quick start example
docs/peppy/code/feature3_derived.md	Updated markdown documentation with new introductory sections and quick start example, plus minor formatting fixes

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