Add Docs Link to Forks Total Storage Size on Usage Quotas Page

What does this MR do and why?

Add a link to the usage quotas page next to the storage size of forks.

Link to the cost factors for forks documentation.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/393580

Screenshots or screen recordings

Before	After

How to set up and validate locally

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

1. Enable namespace storage limits.

2. In a rails console, enable the namespace_storage_forks_cost_factor feature flag:

[1] pry(main)> Feature.enable(:namespace_storage_forks_cost_factor)

3. Navigate to http://localhost:3000/admin/application_settings/namespace_storage as an administrator and set the cost factor for forks to something lower than 1. A good value to use is 0.008.

4. Visit the Usage quotas page. The link should be visible next to the Total storage size of any fork with a cost factor. (private forks in free namespaces do not get a cost factor; any other fork will.)

MR 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 MR.