feat: Shared link bar partial for operator index pages

Operator index pages define their links as page attributes and include
ROOT:partial$operator-link-bar.adoc; the bar itself is maintained here
instead of copy-pasted in 13 repos. Adds an optional Stackable Hub
entry linking to the operator's component page on
https://hub.stackable.tech.

The Feature Tracker URL moves to antora.yml (feature-tracker), next to
a new hub-base-url attribute, so future URL changes are one line here
instead of one PR per operator repo.

Author: lfrancke

antora.yml

@@ -29,6 +29,10 @@ asciidoc:
     end-of-life: false
     # use the attributes below to link to the CRD docs
     crd-docs-base-url: "https://crds.stackable.tech"
+    # links shared by the operator index pages, see the
+    # operator-link-bar.adoc partial in the ROOT module
+    feature-tracker: "https://features.stackable.tech/unified"
+    hub-base-url: "https://hub.stackable.tech"
     crd-docs: "{crd-docs-base-url}/{crd-docs-version}"
     # to make attributes accessible to the UI template, they need to
     # be prefixed with "page-"

modules/ROOT/partials/operator-link-bar.adoc

@@ -0,0 +1,18 @@
+// Shared link bar for operator index pages.
+//
+// Usage in an operator's index.adoc:
+//
+//   :github: https://github.com/stackabletech/airflow-operator/
+//   :crd: {crd-docs-base-url}/airflow-operator/{crd-docs-version}/
+//   :hub: {hub-base-url}/components/airflow
+//
+//   include::ROOT:partial$operator-link-bar.adoc[]
+//
+// {feature-tracker} is defined centrally in antora.yml. Every entry is
+// optional: unset an attribute on the page (e.g. :!feature-tracker: for the
+// internal operators) and its entry disappears.
+[.link-bar]
+ifdef::github[* {github}[GitHub {external-link-icon}^]]
+ifdef::feature-tracker[* {feature-tracker}[Feature Tracker {external-link-icon}^]]
+ifdef::crd[* {crd}[CRD documentation {external-link-icon}^]]
+ifdef::hub[* {hub}[Stackable Hub {external-link-icon}^]]