Remove GLFM spec guide

What does this MR do?

The GLFM snapshot tests and all related code will be deleted soon.

MRs in this sequence of removal:

• Remove wysiwyg/prosemirror glfm specs (!160563 - merged)
• Remove glfm-verify CI job and related tests (!162154 - merged)
• Remove GLFM spec guide (!162743 - merged) (👈 this MR)

Future MRs will be split from !160852

Why are we removing GLFM specs?

In summary, GLFM specs, while a good initiative, no longer serve their purpose. They are slow, redundant and unnecessarily complicated. We already have sufficient coverage at the unit and integration test levels for our Markdown parser and serializers:

• Parser specs: link
• Serializer specs: link
• Markdown API feature specs: link
• Rich text editor feature specs: link

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• 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.