Refactor docs related to Scylla Manager integration

[rzetelskik]

What should the feature do?

We should refactor the pages of our docs related to Scylla Manager integration.

/kind documentation

What is the use case behind this feature?

Currently, the pages of our docs related to Scylla Manager are all over the place. There is an aggregated page Deploying Scylla Manager on a Kubernetes Cluster covering the deployment and task scheduling. Then, in an entirely different place, there's Restore from backup covering the manual procedure for backup restore.

I propose we refactor this part of our docs by creating a separate subtree dedicated to Scylla Manager integration, covering the following in separate pages:

• current architecture of Scylla Manager in k8s clusters
• deploying Scylla Manager in k8s clusters
• scheduling backups
• scheduling repairs
• manual restore from backup procedure

Possibly also additional documents (or integrated into the above) for manager integration with multi-datacenter ScyllaDB clusters.

Alternatively, we can wait with this until we implement proper isolation and managed deployments.

Anything else we need to know?

The purpose of this issue is first and foremost to serve as a place for suggestions and discussion related to Scylla Manager integration docs.