fix: parse one template file at once

[wrenix]

otherwise it is not possible to overwrite a define on the next step.

maybe fix #277

[Makeshift]

Unfortunately this won't build. I was hoping it would fix the overwrite issue I was having.

pkg/document/template.go:390:11: cannot use "" (untyped string constant) as []string value in return statement
pkg/document/template.go:396:53: cannot use []byte(defaultDocumentationTemplate) (value of type []byte) as []string value in argument to append