Add documentation on using CockroachDB database for Trillian

[vipulagarwal]

Trillian is an important part of the Sigstore project. It is an essential component on which Fulcio and CTLog depends. The Sigstore documentation is missing information on configuring Trillian to make use of CockroachDB which essentially provides better scalability for a on-prem (self-hosted) Sigstore instance.