feat(docs): add catalog contributing guide and rename skill

Add mintlify docs page for contributing blocks/components to the
registry catalog. Rename skill from contribute to contribute-catalog
for clearer intent.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/contribute-catalog/SKILL.md

@@ -1,5 +1,5 @@
 ---
-name: contribute
+name: contribute-catalog
 description: Author a new HyperFrames registry block (caption style, VFX block, transition, lower third) or component (text effect, overlay, snippet) and ship it as an upstream PR to the hyperframes repo. Use ONLY when the user wants to CONTRIBUTE to the public catalog — for in-project caption/transition authoring use the `hyperframes` skill, for installing existing registry items use the `hyperframes-registry` skill.
 ---