Feature/5x breaking changes docs (#2491)

* started on NEST breaking changes documentation

* 2x code was rendered in place of 5x code

* cancellationtoken changes does not need to show code, just where it was added

* cancellationtoken changes does not need to show code, just where it was added

* no need to show code for all the readonly changes

* started on manually annotating the breaking changes

* removed code from removal section and reordered docs

* manually categorized more changes

* small touchups and breaking changes preface

* low level breaking changes API

* add breaking changes pointer file

* final small tweaks: removed todo and changed ascii section title to be unique

* typos ty @gmarz @russcam

Author: Mpdreamz