A certificate of trust is a document that outlines the "basics" of your 401k trust, including:
Who is the trustee?
When was trust formed?
What state was the trust formed in?
Any specific rules regarding trustee powers and investments
Most bank and investment sponsors don't need the certificate of trust. Simply providing your "Trust Agreement" (found in your Solo 401k plan package) should suffice.
However, certain banks or investment sponsors may require the Certificate of Trust in addition to your Trust Agreement and/or Adoption Agreement.
The certificate of trust is provided you as a courtesy and is not part of your "official" 401k plan package.
You don't need to sign or notarize unless that's requested by a bank or investment sponsor.