docs: improve hello world

[shahules786]

No description provided.

[greptile-apps]

Greptile Summary

This PR enhances documentation across experimental features and backend implementation guides. The changes focus on two main areas:

1. Backend Documentation Consistency:

   • Standardizes terminology in the backends README by replacing 'my_storage' with 'my_backend'
   • Updates example code snippets in configuration and usage sections to reflect consistent naming

2. Experimental Framework Documentation:

   • Introduces Evaluation-Driven Development (EDD) as a core concept
   • Revamps the hello world example with clearer step-by-step instructions
   • Transitions from numeric to discrete metrics using pass/fail values
   • Improves installation guidance including local backend setup
   • Adds visual aids (hello_world.gif) for better understanding

Confidence score: 5/5

1. This PR is very safe to merge as it only contains documentation improvements.
2. The changes are well-thought-out, maintain consistency, and improve clarity without affecting runtime behavior.
3. While both files have been improved, reviewers may want to give extra attention to docs/experimental/index.md to ensure the new EDD concept is accurately represented.

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