doc: explain how to write ooni nettests

[bassosimone]

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: Publish documentation explaining how to write OONI tests ooni.org#361 and webconnectivity: write prototype of new experiment  probe#1733.
• related ooni/spec pull request: N/A

Location of the issue tracker: https://github.com/ooni/probe

Description

This is a preliminary commit and requires more work. I am going
to push this commit so others soon can start to review it.

This draft supersedes #505

This diff also contains a library of building blocks that is
instrumental to complete websteps. In fact, the tutorial I have
written here will eventually guide people to write their own
implementation of websteps.

I have not written any test for the new code so far. It is
also a stretch to call this new code. It is existing code that
has been written by me or @kelmenhorst that I have adapted
for the purpose of writing the tutorial.

The nice exercise of writing a tutorial is that you are more
easily frustrated if the code is difficult to use XD.

How to review the tutorials

Navigate to internal/tutorial/measure and follow the links
inside the README.md file. Or go to https://github.com/ooni/probe-cli/tree/WORK/internal/tutorial/measure
and read starting from there, which is equivalent.

Editing is quite tricky. Most text comes from comments in
Go files. You need to edit the comments and then you need
to regenerate all the files. The README.md of the
internal/tutorial/measure tutorial contains instructions
on how to do that.

What remains to be done before merging

Mainly adding tests and making sure I didn't lose
anything when adapting existing code. I like the interface
of the new code more, but existing code written last
August may have a better implementation of corner cases.

[bassosimone]

I flagged a couple of places that were wrong. After re-reading, it seems to me all chapters could benefit from more subsections with titles. This will allow to provide comments before and after snippets of code with a more clear delimiter.

[bassosimone]

Superseded by #506