doc: publish the step-by-step design document (#814)

This pull request publishes the step-by-step design document that I have been discussing with @hellais and @DecFox recently. Compared to the document that was approved, this one has been edited for readability.

While there, I figured it was also be beneficial to publish the few ooni/probe-cli related design documents we produced in the past, because they probably help someone to get acquainted with the codebase.

Reference issue for this pull request: https://github.com/ooni/probe/issues/2148

pkg/oonimkall/doc.go

@@ -9,6 +9,10 @@
 // reimplementation of the mkall API implemented by Measurement Kit
 // in, e.g., https://github.com/measurement-kit/mkall-ios.
 //
+// Design
+//
+// See docs/design/dd-001-oonimkall.md in the probe-cli repository.
+//
 // Semantic versioning policy
 //
 // This package is public for technical reasons. We cannot use `go