docs(ci): deduplicate doc images before packaging the artifact

The HTML build copies every referenced image into each language tree, so
the built doc tree is ~324MB with ~227MB of duplicate images. Add a CI step
that collapses byte-identical images into a shared image/ tree and rewrites
the refs before the tar step, dropping the published tarball's tree to
~118MB. The pass lives in .github/scripts/dedup-docs.py, runs only in CI,
and leaves the build itself untouched. It self-verifies that every image
reference still resolves and is idempotent.

Author: grandixximo