Add deployment docs (#82)

* Add deployment docs

Fixes #72

* Add prerelease note

Author: nightkr

docs/antora.yml

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+---
+name: home
+version: "nightly"

docs/modules/cockpit/pages/index.adoc

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+= Stackable Cockpit
+
+This is a visual dashboard to monitor and control Stackable Data Platform clusters.
+
+NOTE: The Stackable Cockpit is currently an early preview, and is not yet a
+      fully supported component of the Stackable Data Platform.

docs/modules/cockpit/pages/installation.adoc

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+= Installation
+
+The Stackable Cockpit is normally installed using Helm.
+
+== Prerequisites
+
+You will need:
+
+* a Kubernetes cluster
+* kubectl
+* Helm
+* htpasswd (from Apache HTTPD)
+
+Resource sizing depends on cluster type(s), usage and scope, but as a starting point we recommend a minimum of the following resources for this service:
+
+* 0.2 cores (e.g. i5 or similar)
+* 256MB RAM
+
+== Authentication
+
+The Stackable Cockpit authenticates users using a htpasswd database. This file can be
+created and maintained using the `htpasswd` tool:
+
+[source,console]
+----
+$ htpasswd -Bc my-htpasswd first-user
+$ htpasswd -B my-htpasswd second-user
+----
+
+NOTE: Stackable Cockpit _only_ supports bcrypt passwords (controlled by the
+      `-B` flag).
+
+== Installing
+
+First ensure that you have installed the Stackable Operators Helm repository:
+
+[source,console]
+----
+$ helm repo add stackable https://repo.stackable.tech/repository/helm-stable/
+----
+
+Then install the Stackable Cockpit:
+
+[source,console]
+----
+$ helm upgrade listener-operator stackable/stackable-cockpit --install \
+  --set-file htpasswd=my-htpasswd
+----
+
+Helm will deploy the service in Kubernetes containers. You're now ready to access the cockpit!

docs/modules/cockpit/partials/nav.adoc

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+* xref:cockpit:index.adoc[]
+* xref:cockpit:installation.adoc[]