Docs: rename Scylla to ScyllaDB

[tzach]

I would like to report an issue in page https://java-driver.docs.scylladb.com/scylla-4.13.0.x/index

(and all pages)

Problem

Rename Scylla to ScyllaDB on headers, where possible (with out breaking code examples!)

[tzach]

@annastuchlik FYI