Document inputs for `include`

Problem

We are working on adding the ability to use include:with: keyword to provide inputs and interpolate values in the included configuration.

As a user who wants to leverage include:with: for my templates I need to know how to use it properly.

Additionally, spec:inputs will also be documented so that it is defined how to use it properly.

Proposal

Document:

• the syntax of the feature and that it works with all types of include
• any limitations
• troubleshooting
• spec:inputs