Publish Feature flag API

What does this MR do?

This MR publishes Feature Flag API as we've tested it out and confirmed deemed stable. Specifically,

• Remove feature flags feature_flag_api from endpoints to allow any users to access the endpoints. (But not enable/disable yet. See #34664 (closed))
• Add documentations.

We also remove feature flag as it's deemed stable.

Related: #9566 (closed)

Screenshots

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

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team