Live preview tool for Boost docs (AsciiDoc & Antora support)

### Description
The goal is to create a live preview tool for Boost documentation that supports both traditional AsciiDoc (built with Boost.Build/b2) and Antora-based docs, using the same toolchains and commands as the actual documentation builds. The tool should render sample docs ("specimens") for each type with their real toolchain, and integrate Antora preview into the existing boostlook_preview.rb script.