Restore [workspace] for non-NDTensors subprojects

Earlier commit dropped [workspace] entirely while removing
NDTensors from it. The non-NDTensors subprojects (benchmark, dev,
docs, examples, test) are sub-environments rather than registered
packages, so the develop-override footgun doesn't apply to them
and the workspace mechanism for shared manifest resolution remains
useful for development.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mtfishman

Project.toml

@@ -3,6 +3,9 @@ uuid = "9136182c-28ba-11e9-034c-db9fb085ebd5"
 version = "0.9.28"
 authors = ["Matthew Fishman <mfishman@flatironinstitute.org>", "Miles Stoudenmire <mstoudenmire@flatironinstitute.org>"]
 
+[workspace]
+projects = ["benchmark", "dev", "docs", "examples", "test"]
+
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"
 BitIntegers = "c3b6d118-76ef-56ca-8cc7-ebb389d030a1"