Add support for Redoc 3 in build-docs command

[larxn]

Problem

When using redocly build-docs to compile my OpenAPI documentation, the output uses Redoc v2. However, when I run redocly preview, I can see my documentation rendered with Redoc 3, which is able to render multiple OpenAPI definitions within the sidebar.

Proposal

A --redoc-version flag (or similar) for the build-docs:

redocly build-docs --redoc-version 3 openapi.yaml

Or maybe a new property in the redocly.yaml?

redocVersion: 3

This would allow users to choose between Redoc 2 and 3.