feat(concerto): remove concerto docs

[mttrbrts]

Closes accordproject/concerto#

Removes old documentation for the Concerto project that is now hosted at https://concerto.accordproject.org

AWS S3 redirects have already been put in place to redirect users, for example:

• https://docs.accordproject.org/docs/model-concerto.html --> https://concerto.accordproject.org/docs/intro/

Author Checklist

• [x] Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• [x] Vital features and changes captured in unit and/or integration tests
• [x] Commits messages follow AP format
• [ ] Extend the documentation, if necessary
• [x] Merging to master from fork:branchname
• [ ] Manual accessibility test performed
  • [ ] Keyboard-only access, including forms
  • [ ] Contrast at least WCAG Level A
  • [ ] Appropriate labels, alt text, and instructions