Simplifications and some fun stuff for the MNIST Gluon tutorial (#13094)

* Simplify mnist Gluon tutorial and add mislabelled sample plotting

* Add mnist Gluon tutorial images

* Gluon MNIST tutorial: Use modern Gluon constructs, fix some wordings

* [Gluon] Move to data loaders and improve wording in MNIST tutorial

* Fix broken links

* Fix spelling of mislabeled

* Final rewordings and code simplifications

* Fix things according to review

- Apply hybrid blocks
- Move outputs outside of code blocks and mark for notebooks
  to ignore
- Remove images, use external link
- Fix a few formulations

* Change activations to sigmoid in MNIST tutorial

* Remove superfluous last layer activations in MNIST tutorial