Make appliance documentation better

[OnGle]

This issue addresses several ways we could improve documentation.

1. include docs/ directory (were relevant) inside appliances (/usr/share/doc/turnkey-$APP_NAME/$TAG/ springs to mind as a candidate)
2. provide an updating system for said docs using turnkey-version it should be trivial to find & pull docs from github.
3. merge tklwebcp docs with docs directory. We could do this by rendering the rst to html, serving said html as static content then embedding it using iframes. This would reduce work on any specific tklwebcp implementation (nodejs & asp.net core for example have their own versions)

As a whole this would reduce work on updating documentation whilst providing a more complex documentation under most cases for end users as well as an offline copy of relevant documentation.