Document templates.Exists

Issue  #552
Add documentation for templates.Exists function with practical example.

content/en/functions/templates.Exists.md

@@ -0,0 +1,37 @@
+---
+title: templates.Exists
+linktitle: ""
+description: "Checks whether a template file exists under the given path relative to the `layouts` directory."
+godocref: ""
+date: 2018-11-01
+publishdate: 2018-11-01
+lastmod: 2018-11-01
+categories: [functions]
+tags: []
+menu:
+  docs:
+    parent: "functions"
+ns: ""
+keywords: ["templates", "template", "layouts"]
+signature: ["templates.Exists PATH"]
+workson: []
+hugoversion: "0.46"
+aliases: []
+relatedfuncs: []
+toc: false
+deprecated: false
+---
+
+A template file is any file living below the `layouts` directories of either the project or any of its theme components incudling partials and shortcodes.
+
+The function is particulary handy with dynamic path. The following example ensures the build will not break on a `.Type` missing its dedicated `header` partial.
+
+```go-html-template
+{{ $partialPath := printf "headers/%s.html" .Type }}
+{{ if templates.Exists ( printf "partials/%s" $partialPath ) }}
+  {{ partial $partialPath . }}
+{{ else }}
+  {{ partial "headers/default.html" . }}
+{{ end }}
+
+```