docs: fix heading ID formatting in develop-with-ai.md

[lacolaco]

Summary

Fix heading ID formatting by removing dots from the anchor ID as per translator guidelines.

Changes

• Fixed heading ID from #providing-context-with-llms.txt to #providing-context-with-llmstxt in adev-ja/src/content/ai/develop-with-ai.md

This follows the heading ID symbol handling rule that removes all symbols except hyphens from anchor IDs.