"Getting started" section across all operators

[fhennig]

We've decided that it would be good for onboarding new users to have a dedicated "Getting started" section in all operators.

It should contain

• an "Install" page with instructions for how to install using either stackablectl or Helm
• a "First steps" page with instructions on how to deploy an instance and verify that it is up, ideally with a simple task to do

This also relates to #204 - we should mention stackablectl in the install page (not done currently)

All the script and yaml files in there should be put into a shell script that can actually be executed, for testing purposes. Have a look at the druid docs, where this has been implemented already: https://github.com/stackabletech/druid-operator/tree/main/docs/modules/getting_started

• Airflow - Docs: Getting Started Guide airflow-operator#142
• Druid - [Merged by Bors] - Getting started docs druid-operator#278
• HBase - Docs: Getting started Guide hbase-operator#227
• HDFS - Docs: Getting started guide hdfs-operator#220
• Hive - Docs: Getting started guide hive-operator#224
• Kafka - Docs: Getting started guide kafka-operator#447
• NiFi - Docs: Getting started guide nifi-operator#325
• OPA - Docs: Getting started guide opa-operator#329
• Spark - WIP - Docs: Getting started guide spark-k8s-operator#113
• Superset - [Merged by Bors] - Add getting started section superset-operator#246
• Trino - Docs: Getting started guide trino-operator#253
• ZooKeeper - Docs: getting started guide zookeeper-operator#536

Secret and Commons Operator don't need a Getting Started guide.

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Acceptance criteria per operator:

• a new module getting_started exists, with an index.adoc, a installation.adoc and first_steps.adoc
• The old installation.adoc is removed.
• All the yaml and shell snippets are in the examples/code directory and can be executed as a script, to test the documentation
• versions to install are templated using the template_docs.sh script and the templating_vars.yaml file.
• Any example files that are incorporated into the getting started guide are removed from the examples directory (because there they are not tested, with the new script they will be tested).

[fhennig]

As part of this, we can also tackle this ticket: stackabletech/issues#239

[fhennig]

Update: The Druid docs were merged.

• All the code & scripts in the Getting Started guide is in an actual script that is testable
• There's a small templating script for templating in the docs now.