Implement glfm_example_status.yml

What does this MR do and why?

Implement partial support for glfm_example_status.yml, and expand documentation.

It also supports preserving existing manual edits to html.yml and prosemirror_json.yml.

This MR implements support for the skip_update_* keys, it does not yet support the skip_running_* keys.

See https://docs.gitlab.com/ee/development/gitlab_flavored_markdown/specification_guide/#glfm_example_statusyml, as well as the updates to that section in this MR (https://gitlab.com/gitlab-org/gitlab/-/blob/caw-glfm-add-example-status-support/doc/development/gitlab_flavored_markdown/specification_guide/index.md#glfm_example_statusyml) for details of how the implementation should work.

How to set up and validate locally

1. Modify glfm_specification/input/gitlab_flavored_markdown/glfm_example_status.yml to change the skip_update_* keys.
2. Make edits to the existing files in spec/fixtures/glfm/example_snapshots/* in skipped and non-skipped entries.
3. Run scripts/glfm/update-example-snapshots.rb.
4. Review changes made to files in spec/fixtures/glam/example_snapshots/*, they should match the changes made to glfm_example_status.yml, and any skipped entries should be preserved. Non-skipped entries should be restored.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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