Add a WIP section, so the doc build doesn't have to fail for new desi…

…gn docs.

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.

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 WIP
+  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   a useful reference for the current system, it moves to section :ref:`design-docs-outdated`.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