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

internal/engine/netx/doc.go

@@ -43,4 +43,7 @@
 //
 // Additionally, new code that does not need to perform measurements
 // SHOULD NOT use netx and SHOULD instead use netxlite.
+//
+// See docs/design/dd-002-nets.md in the probe-cli repository for
+// the design document describing this package.
 package netx