Docs: Add IsPreview to document context

[EngincanV]

If we created the app template by the above command (in the screenshot), it'll create the cached-template (4.2.2 in this case). So we need to add IsPreview variable into the document context, in this way we can specify the --preview option in our documents by adding the following command to where it's needed. {{if IsPreview}} --preview{{end}}