Move and combine snippets for Memory<T> guidelines

[gewarren]

• Move snippets to live near article.
• Add "in-article" index to rules and make them H3s instead of bolded.

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Internal previews

📄 File	🔗 Preview link
docs/standard/memory-and-spans/memory-t-usage-guidelines.md	Memory<T> and Span<T> usage guidelines

[BillWagner]

This LGTM. I had one formatting suggestion if you want to take it.

[gewarren]

Thanks @BillWagner, good suggestion.