Use images to show how GFM is rendered in markdown.md
Since we cannot use GitLab Flavored Markdown (GFM) outside of GitLab, the docs site doesn't render it correctly https://docs.gitlab.com/ee/user/markdown.html#gitlab-flavored-markdown-gfm.
Although I've put notes to each and every GFM feature, users seem to not notice them. There have been at least 3-4 issues opened the last couple of months about this.
Since the GFM is unlikely to be extracted and be able to be used outside of GitLab, the next best thing we can do is replace all GFM rendered content into images in markdown.md.
Edited by Achilleas Pipinellis