#11525: docs: Add LiteLLM provider documentation

## Summary Add documentation for using LiteLLM as a model provider in OpenClaw. ## What LiteLLM provides - Unified API to access 100+ LLM providers (OpenAI, Anthropic, Google, Mistral, etc.) - Automatic failover between models - Cost tracking across providers - Load balancing across API keys ## Changes - New file: `docs/providers/litellm.md` - Updated: `docs/providers/index.md` (added LiteLLM to provider list) ## Why this matters OpenClaw users get access to a broader model ecosystem through a single configuration. LiteLLM handles provider-specific quirks, so agents can focus on their tasks rather than API differences. --- Links: - LiteLLM Docs: https://docs.litellm.ai - LiteLLM GitHub: https://github.com/BerriAI/litellm <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a new provider doc page for LiteLLM (`docs/providers/litellm.md`) and links it from the provider index (`docs/providers/index.md`) so users can discover LiteLLM as an OpenAI-compatible proxy option. <h3>Confidence Score: 3/5</h3> - This PR is mostly safe to merge but contains doc instructions that appear non-functional if copied verbatim. - Changes are documentation-only, but the current LiteLLM page includes an `openclaw onboard --auth-choice litellm-api-key` command and config snippets that don’t appear to match existing OpenClaw onboarding/config conventions, so users following the docs may be blocked. - docs/providers/litellm.md <!-- greptile_other_comments_section --> <!-- /greptile_comment -->