Add pre-commit.ci service to CI/CD section

[dupuyarc]

Requirements

• [ X ] This is Software as a Service not self hosted
• [ X ] It has a free tier not just a free trial
• [ X ] Pricing information is clearly visible without signup or phone calls
  • https://github.com/marketplace/pre-commit-ci
• [ X ] The submission mentions what is free
• [ X ] The submission is not already present in the list
• [ X ] The service has contact details of those running it and a privacy policy
  • pre-commit ci, LLC (postal and email addresses)
  • https://pre-commit.ci/privacy_policy.html

[dupuyarc]

Thank you for maintaining this awesome list!