Completion docs for review

[hghalebi]

1. Architecture Documentation (mod.rs)

Added:

• ASCII diagram showing abstraction layers
• Clear explanation of Prompt, Chat, and Completion traits
• "When to use" guidance for each abstraction level
• Provider-agnostic design examples

Impact: Developers now understand the system architecture at a glance.

2. Common Patterns Section (All Files)

Added to mod.rs:

• Error handling with retry logic
• Streaming responses
• Building conversation history
• Exponential backoff examples

Added to message.rs:

• Building conversation history
• Multimodal messages (text + images)
• Working with tool results
• Performance tips

Impact: Real-world usage patterns are now documented.

3. Troubleshooting Guide (message.rs)

Added:

• "Media type required" error solutions
• Provider capability table
• Large image handling tips
• Builder pattern common mistakes
• Compile_fail examples showing wrong approaches

[joshua-mo-143]

changes to be made:

• Please move all .md files to a folder in the top level of the project (maybe something like "prompts"?).
• Delete whatever .md files you don't actually need for feeding into a prompt.

Rest looks OK... I think. We might need to move some of it into the docsite, there is a lot here.