Add "Getting Started" guide for KerasNLP (#1159)

* Created using Colaboratory

* Convert notebook to py

* Fill into intro block

* Change name of guide

* Created using Colaboratory

* Address comments

* Update to `MaskedLM`

* Respond to comments

* Add link to guide on `/keras_nlp`

* Respond to comments 2

* Remove last hard link

* Serve images from repo

* Remove extra line

* Fix intro to match package README

* Serve images from gcp bucket

* Respond to comments

* Copyedits

* Add generated files for KerasNLP getting started

Co-authored-by: François Chollet <[email protected]>
Co-authored-by: Matt Watson <[email protected]>

Author: 3 people