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

[Rob--W]

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