add missing atom definition

hugo.toml

@@ -131,6 +131,19 @@ defaultContentLanguageInSubdir = false
   taxonomy = ['html', 'rss', 'atom']
   term = ['html', 'rss', 'atom']
 
+[mediaTypes."application/atom+xml"]
+  suffixes = ["xml"]
+[outputFormats.Atom]
+  # https://validator.w3.org/feed/docs/atom.html#whatIsAtom
+  name = "Atom"
+  mediaType = "application/atom+xml"
+  baseName = "atom" # generated file = <baseName>.<mediaType."application/atom+xml".suffixes[0]> = atom.xml
+  isPlainText = false
+  rel = "alternate"
+  isHTML = false
+  noUgly = true
+  permalinkable = false
+
 [server]
   [[server.headers]]
     for = '/**'