Add Bourse to the OpenAI-compatible providers docs

Bourse resells unused AI capacity from the providers at 11-58% under list price, over the OpenAI Chat Completions format. Aider works with it today through the openai-compat path, no code change needed.

export OPENAI_API_BASE=https://api.bourse.run/v1
export OPENAI_API_KEY=<bourse key>
aider --model openai/claude-opus-5

Catalogue and prices: https://bourse.run API guide: https://bourse.run/api-guide

Opening this in case it is worth a mention alongside the other OpenAI-compatible providers. Happy to send the PR if you would take one.