Write a guide that describes how to write (multi-code) examples in the docs

[iwahbe]

We should write down how we currently process code blocks in schemas.

• This should mention how the registry processes <!--Start PulumiCodeChooser --> and {{% examples %}}.
• This should also mention how this interacts with SDK generation, since they do something to strip out non-appropriate code blocks.
• The guide is a “state of the world”, but not intended for external consumption.
• This should be a step before getting us to a better state on docs publishing.

We intend to put together a more intentional way to write multi-language docs (pulumi/pulumi#16099).