Reorganize tutorials into a section

[rossturk]

While reviewing the tutorials, I noticed that there were actually three tutorials spread across two pages:

1. the main tutorial at tutorial.rst
2. the example pipeline that is quietly nestled inside the Testing section of the main tutorial
3. the taskflow tutorial

Both 1 and 3 are currently living at the top-level of the doc structure. This PR splits these three tutorials into separate pages and puts them into a new top-level section called Tutorials.

(I think that these tutorials could use a bit more attention, I intend to spend more time here...ideas welcome.)

Signed-off-by: Ross Turk ross@rossturk.com

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[potiuk]

This is a fantastic start. It's somethign that is badly needed.

It's much better. I've added some comments where I think things can be improved (And why I think like that) as we are touching this one.

[potiuk]

Looking really cool now :)

[potiuk]

🎉