Improve the extension documentation

[maxkfranz]

Issue type

Feature request

Description of new feature

Improve the documentation for extension authors. There should be a description of how to write an extension, rather than relying on a scaffolder.

Motivation for new feature

• The scaffolder incurs a higher maintenance cost than a text description.
• An extension author may want to use a different set of tools, which he prefers.
• A text description is more future-proof than the scaffolder.