doc(enginenetx): add design document (#1595)

This design document documents the current implementation in light of
the changes requested in the #1552
pull request review. The actual changes have been implemented by
previous pull requests and basically boil down to ensure we give the DNS
the priority when dialing.

See #1552 for the original design
review as well as for a list of all the subsequent pull requests that
were merged to address the review comments.

Additionally, this PR explains in the design document what are the
current limitations and what we could do next.

With the merging of this PR, we can close
ooni/probe#2704.

Closes #1552.

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Co-authored-by: Arturo Filastò <arturo@filasto.net>