Move design doc outside public docs

Author: maxrjones

docs/design/chunk-grid.md design/chunk-grid.mddocs/design/chunk-grid.md renamed to design/chunk-grid.md

@@ -77,8 +77,6 @@ nav:
         - Creation sub-module: api/zarr/deprecated/creation.md
   - release-notes.md
   - contributing.md
-  - Design documents:
-    - design/chunk-grid.md
 watch:
   - src/zarr
   - docs