Add proxy config note to usage statistics docs

What does this MR do?

Previously the usage statistics documentation did not mention proxy configuration. Proxy mis-configuration is a common cause of usage statistics failing to be reported, which can cause other features (like DevOps Score) to be non-functional. This adds a brief note about proxy configuration to the top of the usage statistics documentation.

Related to Issue #19475 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content