`handwired/symmetric70_proto`: add `keyboard.json`

[fauxpark]

Description

⚠ handwired/symmetric70_proto/promicro: Build marker "keyboard.json" not found.

Types of Changes

• [ ] Core
• [ ] Bugfix
• [ ] New feature
• [x] Enhancement/optimization
• [x] Keyboard (addition or update)
• [ ] Keymap/layout/userspace (addition or update)
• [ ] Documentation

Issues Fixed or Closed by This PR

Checklist

• [x] My code follows the code style of this project: C, Python
• [x] I have read the PR Checklist document and have made the appropriate changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] I have tested the changes and verified that they work and don't break anything (as well as I can manage).