Docs: Add GraphQL Best Practices section within Frontend GraphQL

What does this MR do?

Adds Best Practices within Frontend GraphQL guide as a place to keep all dos and don'ts for GraphQL FE.

Does this MR meet the acceptance criteria?

Conformity

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• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

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• 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