Add design doc section to homepage

As part of being a more transparent community, we wanted to create a
place to publicly review design docs of larger features before
implementing them.

This gives community members a chance to participate in the design
process and to raise concerns if there are any.

This commit adds an index page as a place for future design docs to
be added.

Change-Id: Ia3eb7fdc677a34856c674dc2808fc3ebc0198eeb
diff --git a/_data/topnav.yml b/_data/topnav.yml
index aa63126..ac5267f 100644
--- a/_data/topnav.yml
+++ b/_data/topnav.yml
@@ -31,5 +31,7 @@
       folderitems:
         - title: Latest
           external_url: https://gerrit-documentation.storage.googleapis.com/Documentation/3.0.0/index.html
+        - title: Designs
+          url: /design-docs-index.html
         - title: Wiki
           external_url: https://gerrit.googlesource.com/homepage/+/master/pages/site/docs/
diff --git a/pages/design-docs/index.md b/pages/design-docs/index.md
new file mode 100644
index 0000000..7e214ce
--- /dev/null
+++ b/pages/design-docs/index.md
@@ -0,0 +1,18 @@
+---
+title: "Design Documents"
+sidebar: gerritdoc_sidebar
+permalink: design-docs-index.html
+hide_sidebar: true
+hide_navtoggle: true
+toc: false
+folder: design-docs
+---
+
+This section houses design documents that were written prior to a system refactoring or the
+introduction of a new feature.
+The list is sorted by recency and links to individual design documents. Review and vetting of the
+documents can be found in Gerrit by using the link inside each doc.
+
+## List of Design Documents
+
+None :-)