fix: add suggestion to 'cannot use... autogenerated' msg (#3309)

[mcomella]

The new output looks like:

UsageError: Cannot use previously auto-generated extension ID {redacted} - This extension ID must be specified in the manifest.json file. For example:

    // manifest.json
    {
        "browser_specific_settings": {
            "gecko": {
                "id": "{redacted}"
            }
        },

    ...
    }

[Rob--W]

Could you drop the example and link to the documentation instead? https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/browser_specific_settings

The documentation already has an example at the top.