docs: Expand LiteLLM backend documentation

[planetf1]

Context

The LiteLLM section in backends-and-configuration.md is minimal. With the WatsonX backend now deprecated (v0.4), LiteLLM is the primary multi-provider path and deserves fuller documentation.

Raised in PR #601 review:

• C10 — comment: Expand section with self-hosted LiteLLM guidance, proxy vs direct usage, configuration examples

Suggested content

1. When to use LiteLLM (multi-provider access, WatsonX migration path)
2. Direct mode vs LiteLLM Proxy — tradeoffs and configuration
3. Self-hosted LiteLLM proxy setup guidance
4. Provider-specific configuration examples (Anthropic, Bedrock, Azure, WatsonX via LiteLLM)
5. Environment variable reference

Files

• docs/docs/guide/backends-and-configuration.md (LiteLLM section)
• Possibly a new docs/docs/integrations/litellm.md if the content is substantial enough

Acceptance criteria

• LiteLLM section covers proxy and direct usage
• At least 2 provider configuration examples
• Migration note for WatsonX users