Create common Cloudflare entrypoint module project

In gitlab-com/content-sites/handbook!13775 (merged) we have defined a set of Terraform modules with a common entrypoint to support the majority of module consumers.

This issue captures the work to create the initial module definition and related infrastructure, and enable related work to progress, including the automated documentation generation in #26929 (closed).

Proposal

• Create a project for the common entrypoint for the Cloudflare module set within the terraform-modules namespace
• Configure a minimal interface and structure for the module to allow for usage
• Configure additional infrastructure (e.g. CI configuration) for the module