Navigation between multiple documentation packages

Hello,

We have many packages with documentation, but the navigation between these packages is not very intuitive. Some people find documentation on Google and they don't know that they should try to check the documentation for a different package as it may contain related information. I think you have to think about what we can do to make users more aware and easier to find the documentation.

I think we can be inspired by the documentation for Terraform. It has a drop-down menu with links to all documentation in the top bar.

They also have links to other documentation packages in the side menu.

It is worth checking if we have references to other documentation packages in the content itself. For example: documentation on [PIP installation](http://airflow.apache.org/docs/apache-airflow/stable/installation.html) has a link to provider packages documentation.

We do not have any reference to `helm-chart` package from the `apache-airflow` package, although it would be helpful on [the Kubernetes page](http://airflow.apache.org/docs/apache-airflow/stable/kubernetes.html).

_Originally posted by @mik-laj in https://github.com/apache/airflow/issues/14644#issuecomment-794613029_