Add Google Tag Manager and Piwik analytics integrations

Originally posted by @jason-warpstream https://github.com/orgs/GitbookIO/discussions/696

What I'm Requesting

Given users cannot add custom JavaScript to global head or body tags of Gitbook and must use official integrations, I'm requesting integrations be investigated (and hopefully) made for Piwik Pro and Google Tag Manager.

Desired Outcome

Ideally, there would be official integrations for both these products where you just have to input your account ID and they work.

The Impact and Why I'm Requesting These

For Piwik:

Many companies use it as an analytics alternative to Google Analytics.
Without Piwik integration in Gitbook, gap exists in scenarios where someone leaves the main website to go to a docs website. Once on the docs website, no data can be sent to Piwik to see how that visitor interacted with Gitbook-hosted docs.
For Google Tag Manager:

This is probably the most-used tag management platform, allowing code snippets to be hosted in a container so that code does not need to be hard-coded into webpages or a product and wait for a release.
Adding support for Google Tag Manager would allow users to integrate other products easily as many SaaS products simply rely on firing a JavaScript snippet and linking it to an account ID.

Start by reviewing the repository's existing integration patterns and the linked GitBook discussion for requirements. Determine how Google Tag Manager and Piwik Pro account IDs would be configured and integrated, then verify that both services work for GitBook-hosted documentation.