Add link to old supported api location

We have external links looking for the API document in designs, which no longer
exists. Add an alias to support this rather than returning 404.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/concepts/supported-api.md

@@ -8,6 +8,7 @@ aliases = [
 	"/old-designs/v2019/supported-api.html",
 	"/old-designs/v2020/supported-api.html",
 	"/old-designs/v2021dev3/supported-api.html",
+	"/designs/supported-api/",
 ]
 outputs = [ "html", "pdf-in",]
 date = "2016-05-28"