Add docs for shortcut fields

What does this MR do?

Update docs to discourage use of 'shortcut fields' (for example, latest_pipeline instead of taking advantage of pagination).

Here's my rationale:

• latest_pipeline and similar fields duplicate existing functionality and need to be maintained as an endpoint (and also may mess up analytics)
• Adding a shortcut field means it creates its own inertia, needs to be deprecated if its dependent field changes
• I suggest we encourage folks to rely on what the framework already gives us, unless there is a compelling reason not to.

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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