Move GLFM example_snapshots under glfm_specification

What does this MR do and why?

Related: Tracking Issue: Implement GLFM scripts per the ... (#361241)

Moves GitLab Flavored Markdown example_snapshot files from spec/fixtures to glfm_specification.

Note that this is an intentional change from the previously documented decision.

This is because, when using these files and scripts in practice, developers have found it less confusing and more convenient to have both the GLFM [specification files] and the example snapshot files colocated under the same top-level glfm_specification directory.

This change has already been approved by @ealcantara and @himkp, who will be the initial primary users of these files.

How to set up and validate locally

1. Run scripts/glfm/update-example-snapshots.rb and ensure there are no failures or unexpected changes (small changes to variable values are still expected).
2. Review documentation changes related to the example snapshots for correctness.
3. Ensure yarn jest spec/frontend/content_editor/markdown_snapshot_spec.js still passes.
4. Ensure bundle exec rspec spec/requests/api/markdown_snapshot_spec.rb still passes.

MR acceptance checklist

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