Add roadmap link to docs

Author: brendan-kellam

docs/docs.json

@@ -110,6 +110,11 @@
                 "href": "https://sourcebot.dev/changelog",
                 "icon": "list-check"
             },
+            {
+                "anchor": "Roadmap",
+                "href": "https://github.com/sourcebot-dev/sourcebot/issues/459",
+                "icon": "map"
+            },
             {
                 "anchor": "Support",
                 "href": "https://github.com/sourcebot-dev/sourcebot/issues/new?template=get_help.md",