Add GitLab-Flavored Markdown (GLFM) Specification Guide to docs

What does this MR do?

• Provides a developer guide for managing a formal specification of GitLab-Flavored Markdown (GLFM)
• Addresses multiple requirements and concerns raised in #345726 (closed)
• ...including the need to create a proper Markdown spec for GLFM: #356440
• Provides an implementation plan for the evolution of the Markdown Golden Master specs and tests to more fully support all requirements.

Related issues

• related navigation MR in gitlab-docs: Add navigation for GLFM Specification Guide (gitlab-docs!2545 - closed)
• related: Redesign Golden Master spec to be markdown-driv... (#345726 - closed)
• related: Create proper Markdown spec for our GFM (GitLab... (#356440)
• related: Rename our "GitLab Flavored Markdown" acronym f... (#24592 - closed)
• related: Implementation MR - Implement GLFM `update-example-snapshots.rb` sc... (!84347 - merged)
• related: Support running GitLab Markdown Golden Master t... (!83249 - closed) (spike implementation, some code from that MR will be reused for this effort)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

Reviewer's checklist

Review App Link: http://main-ee-84220.35.193.151.162.nip.io/ee/development/gitlab_flavored_markdown/specification_guide/

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.