Add BoxyHQ to the repositories.toml

[NathanTarbert]

Hi, thank you for creating a place where folks can go to contribute to open-source. Here is the BoxyHQ issues link - https://github.com/boxyhq/saas-starter-kit/issues

Thanks so much!

ℹ️ Repository information

The repository has:

• [x] At least three issues with the good first issue label.
• [x] At least 10 contributors.
• [x] Detailed setup instructions for the project.
• [x] CONTRIBUTING.md
• [x] Actively maintained.