Update services-enable-runtime.md to clarify Runtime is part of App Builder

[gribbletog]

Added clarification that Runtime is part of App Builder and included a link to the product description.

Content change to provide clarity to customers

Related Issue

Motivation and Context

How Has This Been Tested?

It's a text change. Yes the link works.

Screenshots (if appropriate):

Types of changes

Not code - text.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ x] I have signed the Adobe Open Source CLA.
• [ x] My code follows the code style of this project.
• [ x] My change requires a change to the documentation. (that is why I'm updating the docs :) )
• [ x] I have updated the documentation accordingly.
• [ x] I have read the CONTRIBUTING document.
• [ x] I have added tests to cover my changes.
• [ x] All new and existing tests passed.