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

markup/org/convert.go

@@ -27,8 +27,7 @@ import (
 // Provider is the package entry point.
 var Provider converter.ProviderProvider = provide{}
 
-type provide struct {
-}
+type provide struct{}
 
 func (p provide) New(cfg converter.ProviderConfig) (converter.Provider, error) {
 	return converter.NewProvider("org", func(ctx converter.DocumentContext) (converter.Converter, error) {