Integrate "how to build document locally" into documentation

[stefancyliax]

@stefancyliax Instructions seem fine for me.

However, I am wondering the README here shouldn't be somehow included in the Docu itself as e.g.

• 4.5 Contributing to the OSI Documentation
  • 4.5.1 Building the Docu Locally
  • 4.5.2 Don't hesitate, there is a review process anyways

Originally posted by @PhRosenberger in #49 (comment)