doc: add a WIP section to the index (#4583)

It is not possible to build without error in the presence of a document
that is not in the index. In order for CS not to fail for PRs that
include a new, not-yet-approved doc, adding an index section where they
can be safely linked.

See:
#4560 (comment)

doc/dev/design/index.rst

@@ -8,7 +8,8 @@ See section :ref:`change-proposal-process` for the overview on the overall
 contribution process.
 
 - **Creation**:
-  Design documents are created from the template :file-ref:`doc/dev/design/TEMPLATE.rst`.
+  Design documents are created from the template :file-ref:`doc/dev/design/TEMPLATE.rst`. While the design is still being discussed, it is linked in the
+  section :ref:`design-docs-wip`
 
   Once discussion on a change proposal converges and a design document is
   approved, it is inserted to section :ref:`design-docs-active`.
@@ -30,6 +31,13 @@ contribution process.
   a useful reference for the current system, it moves to section :ref:`design-docs-outdated`.
 
 
+.. _design-docs-wip:
+
+WIP
+===
+.. toctree::
+   :maxdepth: 1
+
 .. _design-docs-active:
 
 Active