Update Agent Builder built-in skills reference for 9.5

Aligns the built-in skills reference with the current skill registry
(allow_lists.ts) in Kibana:

- Add the 9.5 built-in skills across Platform, Streams and significant
  events, Security, Observability, and Elasticsearch
- Handle renames and consolidations cumulatively with applies_to badges
  (streams-exploration to streams-management; hybrid-search,
  semantic-search, and vector-database consolidated into
  search.vector-hybrid-search)
- Move workflow-authoring to GA in 9.5 with an inline preview 9.4 note
- Collapse each skill's assigned tools into a dropdown to reduce noise
- Fix a stale skill example link in skills.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: leemthompo