Page suggestion: User interface

# Short Description of the issue

There are no user-facing documentation pages for the user interface.
I would have expected there to be a guide and reference documentation for the User Interface.

Instead, it is hidden in the [HTC](https://docs.egi.eu/users/tutorials/adhoc/htc-job-submission/#deploying-an-ui) tutorial.

# Summary of proposed changes

I suggest adding some pages for the user interface, and guides for provisioning it in various environments. We need a bit more background information and detail on what is actually done.

There are a few methods described -- Ansible role, Terraform OS, etc. These should be a bit more formalised, with clear paths for users to walk, based on their context, and clear outcomes.

I see these as [how-to-guides](https://diataxis.fr/how-to-guides/).

We also have no ([reference](https://diataxis.fr/reference/) information. We should have a page for that.