Create Gateways realm installation instructions

[jsperhac]

Description

Added markdown describing Gateways realm. This is documentation only and should accompany PR #1411.

Motivation and Context

Provide brief documentation on, and instructions for installation of, this realm.

Tests performed

Markdown viewer.

Checklist:

• The pull request description is suitable for a Changelog entry
• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request

[jpwhite4]

Also please can you update the command listing to match the documentation conventions:

https://open.xdmod.org/9.0/docs-conventions.html

[jpwhite4]

Also do we want a "Gateways realm (alpha)" entry in the menu? I think yes.

[jsperhac]

Also do we want a "Gateways realm (alpha)" entry in the menu? I think yes.

Sounds good. I will add this under the "Installing" heading.