Create a reference page about {{block}}s - name it Block definition.

Should be something similar as the page Template definition.

Not yet sure about its location ... wondering whether it belongs to Templating.
See git repo for examples of block definitions.

Explain

• which files are required.
• the "content model" (node types)
• different possibilities of block definition
  • RichTextBlock
  • FieldSetBlockDefinition
  • etc.