Expose created_at to billable members API

What does this MR do?

Closes #330827 (closed)

This MR adds the Member#created_at field to be exposed to the Billable members API. It also updates the documentation accordingly.

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Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

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• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #330827 (closed)