Improved shareable ASCII

Sharpen RigMark's agentic positioning

Position RigMark against flattering synthetic throughput tests and make
its completed-work contract obvious at first glance.

Replace the flat terminal table with a stronger workload receipt and add
an original, metadata-free hero image for repository and social sharing.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>