Add robots.txt template to disallow robots

See: https://github.com/spf13/hugo/blob/master/docs/content/extras/robots-txt.md

config.toml

@@ -5,6 +5,7 @@ languageCode = "en-us"
 title = "CGSpace Notes"
 newContentEditor = "vim"
 theme = "hugo-theme-bootstrap4-blog"
+enableRobotsTXT = true
 
 [permalinks]
   # use 2016-09 as slug, since my posts are always monthly

layouts/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+
+{{range .Data.Pages}}
+Disallow: {{.RelPermalink}}{{end}}