docs: move ai-coding-assistants under user-guide/

The page was sitting at the top level of docs/source/ while every other
page in the USER GUIDE toctree lives under docs/source/user-guide/.
Move the file, update the toctree entry, and update the absolute URL
in llms.txt to match the new path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/index.rst

@@ -78,7 +78,7 @@ Example
    user-guide/configuration
    user-guide/sql
    user-guide/upgrade-guides
-   ai-coding-assistants
+   user-guide/ai-coding-assistants
 
 
 .. _toc.contributor_guide:

docs/source/llms.txt

@@ -5,7 +5,7 @@
 ## Agent Guide
 
 - [SKILL.md (agent skill, raw)](https://raw.githubusercontent.com/apache/datafusion-python/main/SKILL.md): idiomatic DataFrame API patterns, SQL-to-DataFrame mappings, common pitfalls, and the full `functions` catalog. Primary source of truth for writing datafusion-python code.
-- [Using DataFusion with AI coding assistants](https://datafusion.apache.org/python/ai-coding-assistants.html): human-readable guide for installing the skill and manual setup pointers.
+- [Using DataFusion with AI coding assistants](https://datafusion.apache.org/python/user-guide/ai-coding-assistants.html): human-readable guide for installing the skill and manual setup pointers.
 
 ## User Guide