updated API.md to reflect dataset/entities changes (#656)

* updated API.md to reflect dataset/entities changes

* Add articles and capitalize Central concepts

* correct hyperlinks for Related APIs in Datasets group

* added audit log actions for entity

* move Datasets group after Submissions group

* remove revisionNumber and id from Dataset object

* update download Dataset endpoint

* added details about implicit behavior of uploading/publishing a form with entities

* Grouping changelog and testing ability to push

* Small edits and changes to section names/links

* Spelling and link fix

* Related datasets renamed to dataset properties

* Filled in Dataset and CSV details

* Changes in response to comments

Co-authored-by: Hélène Martin <[email protected]>
Co-authored-by: Kathleen Tuite <[email protected]>