add YAML schema and autocomplete snippet for detections

[devhhu]

Summary

Adds YAML schema and autocomplete snippets to simplify detection authoring and remove reliance on the baked in contentctl templates found when you run contentctl new.

Changes Included

• Added detection.schema.json for detection rule validation.
• Added detection-snippets.code-snippets to provide VSCode autocomplete.
• Added settings.json for built-in YAML extension integration.
• Added basic README guidance on enabling schema validation.

Why

• Helps detection authors working in non-Windows environments (e.g., GCP, macOS) avoid contentctl hardcoding issues, which are found when you attempt to create a detection using contentctl new.
• Reduces friction by providing in-editor autocomplete suggestions.

Notes

• Files are under /docs/yaml-spec/schema.

[devhhu]

Hey team! Just a quick note:

This PR doesn’t change anything in the core content - it just adds some optional docs and some editor tips (YAML schema + VSCode autocomplete). We’ve started using it internally and found it super helpful for making detections more easily, especially outside the default workflow.