Improve boilerplates so they can now contain shortcodes (#3019)

.spelling

@@ -540,6 +540,7 @@ Keycloak
 OpenID
 Istioctl
 Boilerplates
+boilerplates
 
  - search.md
 searchresults

boilerplates/en/example.md

@@ -1,2 +0,0 @@
-This is some boilerplate *markdown* _text_.
-

content/about/contribute/writing-a-new-topic/index.md

@@ -450,8 +450,8 @@ which results in:
 
 {{< boilerplate example >}}
 
-You supply the name of a boilerplate file to insert at the current location. Boilerplates are
-located in language-specific subdirectories of the `boilerplates` directory. Boilerplates are just
+You supply the name of a boilerplate file to insert at the current location. Available boilerplates are
+located in the `boilerplates` directory. Boilerplates are just
 normal markdown files.
 
 ## Using tabs

content/boilerplates/example.md

@@ -0,0 +1,5 @@
+This is some boilerplate *markdown* _text_.
+
+{{< text plain >}}
+A sample nested text block.
+{{< /text >}}

content/boilerplates/index.md

@@ -0,0 +1,3 @@
+---
+headless: true
+---

layouts/shortcodes/boilerplate.html

@@ -1,4 +1,26 @@
-{{- /* Inserts a boilerplate markdown file into the document. See https://preliminary.istio.io/about/contribute/writing-a-new-topic/#embedding-boilerplate-text for details */ -}}
-{{- $path := printf "boilerplates/%s/%s.md" .Site.Language.Lang (.Get 0) -}}
-{{- $markdown := readFile $path -}}
-{{- markdownify $markdown }}
+{{/* This will try to find a resource in the "boilerplates"  top-level content directory */}}
+{{- $name := .Get 0 -}}
+{{- $position := .Position }}
+{{- if $name -}}
+	{{- $bundle := $.Site.GetPage "page" "boilerplates" -}}
+	{{- with $bundle -}}
+		{{- $pattern := printf "%s*" $name -}}
+		{{- $resource := $bundle.Resources.GetMatch $pattern -}}
+		{{- with $resource -}}
+			{{- .Content | safeHTML -}}
+		{{- else -}}
+			{{/* It is not a resource in the top-level boilerplates directory. Try to find the page relative to the current page instead. */}}
+			{{- $path := path.Join $.Page.Dir $name -}}
+			{{- $page := $.Page.Site.GetPage "page" $path -}}
+			{{- with $page }}
+				{{ .Content }}
+			{{- else -}}
+				{{ errorf "Could not find %s (%s)." $pattern $position }}
+			{{- end -}}
+		{{- end -}}
+	{{- else -}}
+		{{ errorf "'boilerplates' direcory was not found (%s)" $position }}
+	{{- end -}}
+{{- else -}}
+	{{- errorf "Missing name in boilerplate (%s)" $position -}}
+{{- end -}}