Fix https://github.com/scylladb/scylladb/issues/11376
This PR adds the upgrade guide from version 5.0 to 5.1. It involves adding new files (5.0-to-5.1) and language/formatting improvements to the existing content (shared by several upgrade guides).
Closes#11577
* github.com:scylladb/scylladb:
doc: upgrade the command to upgrade the ScyllaDB image from 5.0 to 5.1
doc: add the guide to upgrade ScyllaDB from 5.0 to 5.1