Create `/for/Cassandra/` page following the MariaDB model

[tartkri-bot]

Goal

Create a new landing page for Cassandra, modeled after the existing MariaDB page at https://openeverest.io/for/mariadb/.

What's needed

• Fill in the markdown template in the correct /content/for/cassandra/ directory.
• Provide annotated screenshots of the Cassandra-related UI flows (provisioning, cluster overview, backups, advanced config).
• Ensure the "Powered by" section references the correct upstream operator for Cassandra.
• Check that all links point to existing OpenEverest docs or the external operator repo.

This is a great first contribution for anyone familiar with the technology.