docs: Add Hugging Face Inference Providers documentation

[davanstrien]

Adds documentation for using Hugging Face Inference Providers with Aider.

Key additions:

• Setup instructions using OpenAI-compatible client
• Model discovery guidance (Hub browsing, model cards, API docs)
• Provider selection for users with existing credits/preferences
• Links to Inference Providers documentation

[CLAassistant]

All committers have signed the CLA.

[davanstrien]

@paul-gauthier I believe this is ready now, but let me know if you need any changes 🤗

[znation]

Hi @paul-gauthier, it would be great to get this merged if it looks good. I ended up figuring it out myself by trial and error and telling my coworkers before discovering this PR.