chore: update .gitignore to exclude GIST_CONTENT.md from future tracking

Add documentation/GIST_CONTENT.md to .gitignore for future modifications.

Note: The file was already committed in the previous commit (a726cc3),
so this only affects future changes to the gist content file.

This allows local gist content updates without affecting the repository,
while keeping the initial version as reference documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitignore

@@ -14,4 +14,5 @@ PROMPTS.md
 .env.development.local
 .env.test.local
 .env.production.local
-medium-content-pro/*
+medium-content-pro/*
+documentation/GIST_CONTENT.md