Docs: Add OpenAI GPT Store integration guide (MemMachine#564)

Add comprehensive guide for onboarding MemMachine into the OpenAI GPT Store, including step-by-step instructions for creating a custom GPT and configuring MemMachine actions.

- Add GPTStore.mdx with detailed setup instructions
- Add 4 screenshot images for visual reference
  - GPTsCreationPage.png
  - GPTsConfigPage.png
  - GPTsAddActionsPage.png
  - GPTsAvailableActionsPage.png

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

Co-authored-by: Claude <[email protected]>

Author: dxu104