[Rough Draft] Composition-API: Introduction (#67)

* Add Composition-API: Introduction draft

* Fix image links

* Add styling for Tips and Warnings. Fix missing `js` syntax flag

* Apply suggestions from An Phan

Co-Authored-By: Phan An <[email protected]>

* Apply suggestions from code review

Co-Authored-By: Phan An <[email protected]>
Co-Authored-By: Rahul Kadyan <[email protected]>

* Convert tabs to spaces

Co-authored-by: Phan An <[email protected]>
Co-authored-by: Rahul Kadyan <[email protected]>

Author: 3 people