Further clarify documentation that is template-only vs documentation that should be included in a repo that uses this template

[rocketnova]

In places, it is unclear when reading the documentation in this repo whether the documentation is for:

1. The template. We want documentation that explains how to use the template and how it works with other templates. The audience for this documentation are the engineer(s) that initially set up the repo, as well as any engineers that might want to pull in changes in the future.
2. The child repo. We want documentation that a child repo can inherit that explains what it inherited and how to use it (e.g. a system architecture diagram, instructions for how to run make commands). The child repo should modify that documentation and make it specific to its project context. The audience for this documentation are the project engineers that onboard to a project after it has been setup.

The confusion between which audience a piece of documentation is for has become increasingly clear while working with users that are brand new to our template ecosystem. We should do another pass at trying to clarify this so that the templates are more approachable for new users.

[doshitan]

Potentially improved by moving the template content (what actually gets installed) into a subdirectory, like this experiment or any of the template-application-* repos.

[doshitan]

Related: #799