Update provider documentation for new providers and baseUrl changes

[bhouston]

Update provider documentation for new providers and baseUrl changes

Description

Based on PR #273, we need to update documentation to include the new providers (gpustack/local-openai and xai) and update references from ollamaBaseUrl to baseUrl.

Changes needed

1. Create documentation for new providers:

   • local-openai.md for GPUStack and other OpenAI-compatible servers
   • xai.md for Grok models from xAI

2. Update references from ollamaBaseUrl to baseUrl in:

   • Ollama provider documentation
   • README.md
   • Other relevant files

3. Update the providers index.mdx to include the new providers

Implementation plan

• Create the new documentation files following the existing structure
• Update all references to ollamaBaseUrl
• Ensure all provider documentation is complete and consistent

Related issues/PRs

This is based on changes made in PR #273, which fixes issues #272, #271, #261, #260, and #148.