docs: add OpenAI-compatible providers section; point profile setup_urls at jcode docs (#89)

The opencode-zen/opencode-go and 9 other built-in OpenAI-compatible
profiles (302ai, baseten, cortecs, firmware, huggingface, moonshotai,
nebius, scaleway, stackit) had setup_url pointing at opencode.ai docs,
which is misleading for jcode users (surfaced in #82).

- Repoint all 11 to https://github.com/1jehuang/jcode#openai-compatible-providers
  (matching the existing deepseek profile).
- Add a '#### OpenAI-compatible providers' README section whose slug is
  exactly that anchor, documenting the built-in named profiles, the
  generic custom-endpoint path, and the JCODE_STREAM_IDLE_TIMEOUT_SECS /
  per-model context_window overrides.

No logic change; only setup_url strings + docs.