Scope extensions update to split "origin" and "site" type extensions (#…

…109)

* Update scope_extensions-explainer.md

Incomplete changes to scope_extensions-explainer.md

* Update scope_extensions-explainer.md

* Update scope_extensions introduction

* Only address "origin" extensions in the existing scope_extensions explainer, leaving "site" extensions to be addressed by a seperate explainer

* Add link to fine-grain scoping discussion issue in w3c/manifest

* Address review feedback