docs: add Cheaper Inference provider guide - #1367

Changed files:

• docs/llm-providers/cheaperinference.mdx (new guide)
• docs/llm-providers/overview.mdx (overview card)
• docs/docs.json (nav entry in "LLM Providers")

Setup shown in the guide:

export STRIX_LLM="openai/gpt-5.4-mini"
export LLM_API_KEY="ci_live_..."
export LLM_API_BASE="https://api.cheaperinference.com/v1"

Checks:

• docs.json parses as valid JSON.
• I checked the API with a live key: model list, chat, streaming, tool calls and JSON schema output with gpt-5.4-mini.
• No trailing whitespace; files end with a newline.
• Docs only. No Python changed, so make check-all does not apply.