docs(stage-router): mark capable_first experimental, and warn at startup (#363)

Every published threshold and routing result comes from efficient_first
runs. capable_first works and the server accepts it, but it has not been
benchmarked, so there is no calibrated confidence_threshold for it and no
measured accuracy or cost to set expectations against.

The docs now list efficient_first first and carry a warning admonition
saying so, and the TOML schema table marks capable_first experimental. The
server logs a warning at startup when a stage_router route selects it, so
the caveat reaches operators who never read the page.

Signed-off-by: Sabhatina Selvam <sabhatinas@nvidia.com>