docs: Use the shared link bar and link to the Stackable Hub

[lfrancke]

Description

Adopts the shared operator link bar from stackabletech/documentation#872: the index page defines its links as attributes and includes ROOT:partial$operator-link-bar.adoc instead of carrying its own copy of the bar. New entry: Stackable Hub, linking to https://hub.stackable.tech/components/airflow. The Feature Tracker link is removed (both from the bar and the "Useful links" section), as decided for the docs overall.

This is my test before rolling it out to all other operators.
I tried it with a local Antora build but I'll need @xeniape 's wisdom for how to handle this with other branches etc.....

[xeniape]

So basically Antora matches the branches defined in here: https://github.com/stackabletech/documentation/blob/main/antora-playbook.yml.
That means, if this change here needs to work on branch 26.3 for example it needs to be added to that branch of airflow-operator as well. But since Antora matches the airflow-operator 26.3 branch with the documentation branch 26.3, the hub-base-url and partial need to be present in the documentation 26.3 branch as well. So the other PRs adding those in documentation main branch, need to be backported.
That's my understanding of it at least.