examples.json should include a way to list significant manifest keys

examples.json lists all samples plus relevant extension APIs in javascript_apis. But there are several extension APIs with significant functionality in its manifest keys. By significant, I mean not just because it is referenced (e.g. name and description are quite meaningless), but because the example is a notably good demonstration of the APIs in use.

To make it easier to find the right examples, we should introduce a new key, e.g. manifest_keys to link examples to manifest keys.

Tasks:

Update CONTRIBUTING to list the new key.
Update examples.json to mention significant manifest keys.
Update macro to parse key in https://github.com/mdn/yari/blob/main/kumascript/macros/WebExtAllExamples.ejs
Update macro to parse key in https://github.com/mdn/yari/blob/main/kumascript/macros/WebExtExamples.ejs
Update articles under https://github.com/mdn/content/tree/main/files/en-us/mozilla/add-ons/webextensions/manifest.json to (automatically) reference the new examples with the macro.

Example of expected results:

Update examples.json to add "manifest_keys": ["protocol_handlers"] to the open-irc-links example at

webextensions-examples/examples.json

Lines 586 to 590 in 9433f84

    
           { 
        
               "description": "Demonstrates the use of protocol handlers.", 
        
               "javascript_apis": [], 
        
               "name": "open-irc-links" 
        
           }

Append {{WebExtExamples}} to ## Examples at files/en-us/mozilla/add-ons/webextensions/manifest.json/protocol_handlers/index.md in the mdn/content repo
The example should be rendered at https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/protocol_handlers#example
The table of examples should also list the manifest key at https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Examples

	{
	"description": "Demonstrates the use of protocol handlers.",
	"javascript_apis": [],
	"name": "open-irc-links"
	}