Make sure every doc project includes the 404 error page

[dgarcia360]

• Update theme: Move 404 page to main theme #58 Updated notfound extension #59
• Main docs: https://github.com/scylladb/scylla-docs/pull/3157
• Scylla Monitoring: Update theme scylla-monitoring#1068
• Scylla Python Driver: Update theme python-driver#71
• Scylla Java Driver: Autogenerate Java docs (II) java-driver#38
• Scylla Care-Pet Example: Update theme care-pet#16
• Mermaid: https://github.com/scylladb/mermaid/pull/2259