Markdown guide: how to embed GitLab Snippets in md files

Marcia Ramosrequested to merge
markdown-guide-embed-gitlab-snippets into master

Adds a section to the markdown guide on how to embed GitLab Snippets.

Review app: /handbook/product/technical-writing/markdown-guide/#embed-gitlab-snippets

@dplanella hope this helps ;)

cc/ @rebecca FYI

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