add Claude context files for docs authoring and prerelease extension

(cherry picked from commit 16e346f)

Author: cderv

.claude/rules/docs-authoring.md

@@ -0,0 +1,16 @@
+---
+paths:
+  - "docs/**/*.qmd"
+---
+
+# Docs Authoring
+
+## Prerelease shortcodes
+
+When documenting a feature introduced in a specific Quarto version, add at the top of the section:
+
+```
+{{< prerelease-callout X.Y >}}
+```
+
+Shows a pre-release callout before X.Y ships, disappears automatically after — never remove them manually. For blog posts use `type="blog"`. For URLs use `{{< prerelease-docs-url X.Y >}}`. See `_extensions/prerelease/` for details.

_extensions/prerelease/CLAUDE.md

@@ -0,0 +1,3 @@
+# Prerelease Extension
+
+See `prerelease.lua` for implementation and #1961 for design rationale.