Add Pages cache configuration settings

What does this MR do?

Adds multiple flags used to configure Pages API-based configuration cache's behavior to the documentation. Examples of modifying these settings are also included.

Related to gitlab-pages#520 (closed).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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• Style guides
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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