disables Google Analytics on docs site (#1113)

This is a follow-up from a private maintainer discussion (https://github.com/orgs/just-the-docs/teams/maintainers/discussions/11?from_comment=12). The cliff's notes for the public are: we believe we are not currently GDPR-compliant in how the docs site itself serves analytics.

For now, we'll disable analytics; we don't use them anyways. We will then:

- add analytics and the relevant banner to the regression test repository
- in the future, workshop a built-in solution for JtD users to be GDPR compliant
- optionally, clarify the role of users of this theme (their responsibility is to make their sites GDPR compliant)(