[WIP] Structurize, update and extend the documentation

[tnozicka]

Description of your changes:
TODO

Which issue is resolved by this Pull Request:
Resolves #1578 (and more)

Requires

• Allow updating Sphinx-Substitution-Extensions and update dependencies sphinx-scylladb-theme#1295

[scylla-operator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tnozicka

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [tnozicka]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[scylla-operator-bot]

@tnozicka: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/docs	b7002d5	link	true	/test docs
ci/prow/docs-multiversion	b7002d5	link	false	/test docs-multiversion
ci/prow/e2e-gke-serial	b7002d5	link	true	/test e2e-gke-serial
ci/prow/e2e-gke-parallel	b7002d5	link	true	/test e2e-gke-parallel
ci/prow/e2e-gke-parallel-clusterip	b7002d5	link	true	/test e2e-gke-parallel-clusterip

Full PR test history. Your PR dashboard.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.