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

resources/page/site.go

@@ -32,6 +32,7 @@ type Site interface {
 	Language() *langs.Language
 	RegularPages() Pages
 	Pages() Pages
+	Home() Page
 	IsServer() bool
 	ServerPort() int
 	Title() string
@@ -89,6 +90,10 @@ func (t testSite) Language() *langs.Language {
 	return t.l
 }
 
+func (t testSite) Home() Page {
+	return nil
+}
+
 func (t testSite) Pages() Pages {
 	return nil
 }