Add Facebook Pixel ID setting

[pestevez]

This PR adds a new configuration option to set the ID of a Facebook Pixel to track page views.

Motivation

Using the Facebook Pixel to track page views in the generated docs site.

Have you read the Contributing Guidelines on pull requests?

Yes.

Test Plan

• Create a Facebook Pixel (or get the ID of an existing one.)
• Update your Docusaurus' siteConfig.js file and add a new field named facebookPixelId to the exported object siteConfig. The value should be the ID of the pixel referenced in the previous step.
  • Example facebookAppId: '352490515235776',
• Launch your docs site generated by Docusaurus by running npm start in the website folder.
• Navigate to the home page e.g. http://localhost:3000/
• Verify the pixel is firing Page View events. These are some of the options:
  • Using the Pixel Helper Chrome plugin.
  • Using the Event Debugger of Facebook Analytics.

[pestevez]

Fixes #503

[JoelMarcey]

Nice. Thanks!