docs: add marketplace add step before plugin install for Codacy Skills

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: claudiacodacy

docs/codacy-cloud-cli/index.md

@@ -45,6 +45,7 @@ The [Codacy Skills](https://github.com/codacy/codacy-skills) let your AI assista
 **Claude Code** (recommended):
 
 ```bash
+claude plugin marketplace add codacy/codacy-skills
 claude plugin install codacy-skills@codacy
 ```