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 https://github.com/mdn/webextensions-examples/blob/9433f842d6295db366c399df752332d78a9cc859/examples.json#L586-L590
• 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

Start with CONTRIBUTING and examples.json in mdn/webextensions-examples, then inspect the WebExtAllExamples.ejs and WebExtExamples.ejs macros in mdn/yari. Review the protocol_handlers manifest documentation in mdn/content as the reference case. Done means significant manifest keys are recorded, both macros parse them, and the relevant documentation pages render the examples and manifest key in their tables.