Clean up and document analytics scripts

What does this MR do and why?

• Removes unused/outdated/duplicative analytics scripts (Facebook analytics and a duplicate copy of Google Tag Manager)
• Loads all analytics code from the analytics.html template (we're removing gtm.html and moving that code into analytics.html)
• Adds the async attribute to scripts that didn't already have it. We do not want these scripts to block
• Removes outdated cookies page (we now use the OneTrust cookie preferences widget, which links to a SSoT cookies page in the handbook)
• Adds documentation about in-use analytics scripts

Closes https://gitlab.com/gitlab-org/technical-writing/-/issues/722

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Compile for production to include analytics: NANOC_ENV="production" make compile
3. Browse a few pages with the browser console open and check for analytics-related errors

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.