[Doc] extensibility framework overview (#2863)

* Add TOC entry

* Add new topic file and outline

* Add how it works section

* Add interceptor pattern section and image

* Add content for Targets section

* Add content for Intercept files

* Finish Declare file content

* Add content for Targets in PWA Studio packages

* Add list of extension examples

* Add extensibility overview diagram

* Apply suggestions from code review

Co-authored-by: Andy Terranova <13182778+supernova-at@users.noreply.github.com>

* Fix links

* Fix linting issues

* Add example intercept file content

Co-authored-by: Andy Terranova <13182778+supernova-at@users.noreply.github.com>
Co-authored-by: Devagouda <40405790+dpatil-magento@users.noreply.github.com>

pwa-devdocs/src/_data/getting-started.yml

@@ -56,6 +56,9 @@ entries:
           - label: Configuration management
             url: /pwa-buildpack/configuration-management/
 
+          - label: Extensibility framework
+            url: /pwa-buildpack/extensibility-framework/
+
           - label: Scaffolding
             url: /pwa-buildpack/scaffolding/