Custom multi-target code blocks in the documentation

Like the following example, let's add a custom code-block renderer to display the example codes in multiple target contexts.

We could also consider the MSDN style, but in our case, individual code blocks may or may not have the full multi-target contents unlike the .NET manual.

For instance, the Client SDK's functional API examples may be presented as:

- Synchronous version (based on `ai.backend.client.session.Session`)\* Some APIs are provided as the async version only.

- Asynchronous version (based on `ai.backend.client.session.AsyncSession`)
- Raw HTTP traces (#1799)

JIRA Issue: BA-167