docs[tutorial]: Making a GraphQL Request (POST/GET basics, fetch)

[moonmeister]

User Story

As a toolkit user, I want to do a tutorial on POST/GET basics with fetch, in order to optimize performance for my project.

Acceptance Criteria

• A public tutorial POST/GET basics using fetch
• Succinctly teaches when and why to use POST
• Succinctly teaches when and why to use GET

Notes

Existing explanation: https://github.com/wpengine/hwptoolkit/blob/main/docs/explanation/get-vs-post.md

This should live in https://github.com/wpengine/hwptoolkit/tree/main/docs, in a tutorial directory if one doesn't exist yet.