Before finishing the ongoing refactoring and leaving whatever
is left of netx in tree, I would like to restructure it so that
we'll have an easy time next time we need to modify it.
Currently, every functionality lives into the `netx.go` file and
we have a support file called `httptransport.go`.
I would like to reorganize by topic, instead. This would allow
future me to more easily perform topic-specific changes.
While there, improve `netx`'s documentation and duplicate some of
this documentation inside `internal/README.md` to provide pointers
to previous documentation, historical context, and some help to
understand the logic architecture of network extensions (aka `netx`).
Part of https://github.com/ooni/probe-cli/pull/396
64bffbd941