Add docs for reducing data transfer costs

What does this MR do and why?

Gitlab recently added details to the pricing FAQ page about data transfer limits. Although these are still only soft limits, GitLab is reaching out to customers with high data transfer costs to better understand their use case and help them to reduce the amount of data they are transferring.

As we reach out to those customers, it would be helpful to share documentation with suggestions on how to remediate the issue.

This MR adds a page to the docs and a few suggestions on how to do so. Also, since it's possible that customers may be transferring more data than expected due to product limitations, I left a section for related issues. This will help folks to find and contribute to known issues.

Related to: https://gitlab.com/gitlab-com-top-initiatives/free-saas/-/issues/50

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