docs(evaluator): fix Open Source SDK link and drop gated benchmarks pages (#1345)

Point the Open Source SDK button on the Evaluate Agents & Models page at
packages/nemo_evaluator_sdk in nemo-platform instead of NVIDIA-NeMo/evaluator.

Delete the evaluator benchmarks pages: they are omitted from
docs/fern/versions/latest.yml, so Fern never built or served them, and no
published page links into them. Their gated-nav.yml block and the two
snippets used only by those pages go with them.

Signed-off-by: Sandy Chapman <schapman@nvidia.com>

Author: SandyChapman