Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

hugolib/page__new.go

@@ -17,6 +17,8 @@ import (
 	"html/template"
 	"strings"
 
+	"go.uber.org/atomic"
+
 	"github.com/gohugoio/hugo/common/hugo"
 
 	"github.com/gohugoio/hugo/common/maps"
@@ -36,7 +38,8 @@ func newPageBase(metaProvider *pageMeta) (*pageState, error) {
 	s := metaProvider.s
 
 	ps := &pageState{
-		pageOutput: nopPageOutput,
+		pageOutput:                        nopPageOutput,
+		pageOutputTemplateVariationsState: atomic.NewUint32(0),
 		pageCommon: &pageCommon{
 			FileProvider:            metaProvider,
 			AuthorProvider:          metaProvider,