Documentation improvements: Tutorials and FAQs

[arno-di-loreto]

It would be a great addition to the spec to have:

• Tutorials from beginner to expert level (disclosure: I'm currently writing one)
• FAQs to find a quick response to a specific use case without having to dig in the spec or do the tutorials ("how do I reuse a definition", "how do I set a response header", ...)
• Standalone example YAML files (linked to tutorials, FAQs)

[arno-di-loreto]

parent OAI/OpenAPI-Specification#589