📖 Use MarkdownSnippets to generate snippets in documentation

[martincostello]

As a follow-up to #3365, use MarkdownSnippets to generate the code snippets in the Markdown documentation.

This will make it easier to keep the documentation up-to-date, particularly with the changes coming for Microsoft.OpenApi 2.0.0 (see #3283).

A good model for this to copy is the one in Polly, such as its Snippets project.

[peter-csala]

I can work on this but not sooner than mid of June.