Summary

Adds missing xAI model provider references to the main model providers section. PR #504 added xAI documentation to the
community section but missed adding it to:

• Model providers navigation in mkdocs.yml
• Model providers index table
• Redirect stub file

Changes

• Add xai.md redirect stub in docs/user-guide/concepts/model-providers/
• Add xAI entry to the providers table in index.md
• Add xAI to mkdocs.yml nav under model-providers (with community badge)

Related

• Follows up on [Community Provider] add xAI model provider documentation #504