Update GLFM Specification Guide with Markdown Specifications Hierarchy and Terminology

What does this MR do and why?

Iterating on a diagram which represents the taxonomy relationships, hierarchy, and terminology for the various markdown specification: https://gitlab.com/gitlab-org/gitlab/-/blob/caw-glfm-markdown-docs-spec-diagram/doc/development/gitlab_flavored_markdown/specification_guide/index.md#official-specifications-vs-internal-extensions.

This is a visual aid and documentation to capture and document the discussions that Brett Walker and @cwoolley-gitlab started.

See related parent issue: #361241

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