Clean up tutorial docs for installable app pattern with flaskr (#2002)

* Clean up tutorial docs for installable app pattern

- reading sequentially through the tutorial works.
- fixes references to `export FLASK_APP=flaskr.flaskr`

* Fixes titles for each section of flaskr tutorial

* Revert grammar

* Emphasize the Packaging Guide

- adds more general packaging resource
- removes the emphasis put on setuptools

* rephrase and remove note admonitions

- expanded on few points
- removed note blocks, they are unneccessary

* Remove note about reinstalling to update cli

- I had mistakenly thought it was necessary to
  re-install the app to update the cli.
- the `--editable` flag detects the change and
  the cli updates without issue.

docs/tutorial/css.rst

@@ -1,6 +1,6 @@
 .. _tutorial-css:
 
-Step 9: Adding Style
+Step 8: Adding Style
 ====================
 
 Now that everything else works, it's time to add some style to the