docs: clarify Ollama and OpenAI-compatible setup

[Beandon13]

Summary

• Clarifies local Ollama setup with the ollama/ model prefix and the /v1 base URL.
• Adds an OpenAI-compatible provider page documenting OPENAI_BASE_URL with the openai/ prefix.
• Updates the README custom base URL example so it uses OPENAI_BASE_URL instead of OLLAMA_API_BASE.

Why

Issue #428 calls out confusion around Ollama, OpenAI-compatible endpoints, and when provider prefixes are required. The updated docs make the routing explicit and give copy-pasteable examples for both normal local Ollama and generic OpenAI-compatible endpoints.

Test plan

• python3 - <<'PY' ... docs sanity checks ... PY
• uvx --with mkdocs-material --with 'mkdocstrings[python]' mkdocs build --site-dir /tmp/cai-docs-build-428

Note: mkdocs build --strict still fails on existing repository-wide warnings unrelated to this change; non-strict build succeeds.

Fixes #428