rewrite tutorial docs and example

docs/tutorial/next.rst

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+Keep Developing!
+================
+
+You've learned about quite a few Flask and Python concepts throughout
+the tutorial. Go back and review the tutorial and compare your code with
+the steps you took to get there. Compare your project to the
+:gh:`example project <examples/tutorial>`, which might look a bit
+different due to the step-by-step nature of the tutorial.
+
+There's a lot more to Flask than what you've seen so far. Even so,
+you're now equipped to start developing your own web applications. Check
+out the :ref:`quickstart` for an overview of what Flask can do, then
+dive into the docs to keep learning. Flask uses `Jinja`_, `Click`_,
+`Werkzeug`_, and `ItsDangerous`_ behind the scenes, and they all have
+their own documentation too. You'll also be interested in
+:ref:`extensions` which make tasks like working with the database or
+validating form data easier and more powerful.
+
+If you want to keep developing your Flaskr project, here are some ideas
+for what to try next:
+
+*   A detail view to show a single post. Click a post's title to go to
+    its page.
+*   Like / unlike a post.
+*   Comments.
+*   Tags. Clicking a tag shows all the posts with that tag.
+*   A search box that filters the index page by name.
+*   Paged display. Only show 5 posts per page.
+*   Upload an image to go along with a post.
+*   Format posts using Markdown.
+*   An RSS feed of new posts.
+
+Have fun and make awesome applications!
+
+.. _Jinja: https://palletsprojects.com/p/jinja/
+.. _Click: https://palletsprojects.com/p/click/
+.. _Werkzeug: https://palletsprojects.com/p/werkzeug/
+.. _ItsDangerous: https://palletsprojects.com/p/itsdangerous/