Add documentation for the Google Ad Manager package

[michalczaplinski]

The code PR: frontity/frontity#522

The PR only implements one component <GooglePublisherTag/> which is implemented according to the Feature Discussion

The main thing to be documented I think is how the user can:

1. Install the package npm i @frontity/google-ad-manager
2. Add and configure it in the frontity.settings.js like:

{
      name: "@frontity/google-ad-manager",
      state: {
        // Fills with ads.
        fills: {
          googleAdManager1: {
            slot: "Below Header",
            library: "googleAdManager.GooglePublisherTag",
            props: {
              unit: "/1234567/sports",
              size: [728, 90],
              id: "div-gpt-below-header",
            },
          },
        }
     }  
   }

[luisherranz]

Whenever you go ahead with this, please also update the README of the package: https://github.com/frontity/frontity/blob/3548df6c62d5c3e05aebd071367d48d79caa7e9d/packages/google-ad-manager/README.md

[mburridge]

Closed by #5