updates to the tutorials

[djarecka]

I've only started the changes - for now I only worked on:

• 1_intro_pydra
• 2_intro_functiontask

I've tried to think about the Diataxis suggestions, mostly about:

• "a tutorial must help a beginner achieve basic competence with a product"
• "tutorials are learning-oriented, and how-to guides are task-oriented"

I've removed several sections thinking that they should be moved to:

• Explanations:
  • the text from 1_intro_pydra
• How-To Guides:
  • second way of annotation the output
  • entire Audit section
  • entire Type Checking section

In addition I added several TODO-LINK to remember about the links from future Explanations (or Installation) sections. I also change "we" to "you" in most of the places.

Please let me know what you think! You can suggest changes in this PR, or if you prefer you can merge it and work on improvement starting from new master (as long as we don't have to resolve conflicts in the notebooks everything works :) )

[yibeichan]

the content looks good to me. a few notes:

• do we want to set up a rough length limit for each notebook in the tutorial/how-to-guide/etc.
• I'm okay with using "we" when explaining things to the user; it feels like "we" are from the same group. we can use "you" when we want the user to try some exercises.