chore(docs): seed shared docs folder for sync

Author: mottych

docs/shared/README.md

@@ -0,0 +1,5 @@
+# Shared Documentation
+
+This folder is synchronized across PurposePath repositories by the Sync Shared Documentation workflow.
+
+Contents here should be generic, reusable specs or shared guidance. Edits on dev will propagate to other repos when the workflow runs.