[Documentation] Write a User Guide. Full docs, incl. side topics.

[mlange-42]

User Guide

Possible sections:

• [x] Yarner syntax and features
• [x] Configuration
• [x] CI/CD examples/templates (separate issue: #50)
• [x] Managing multi-file projects: recommendations for project structure under diffenent use cases
• [x] Markdown tool recommendations
  • [x] MarkText MD editor
  • [x] Atom
  • [x] Math formulas: RStudio as MD editor
  • [x] For GitHub, see A hack for showing LaTeX formulas in GitHub markdown.md
  • [x] Conversion tool: Pandoc (to HTML, PDF, DOCX, ...)
• [x] Usage examples
  • [x] mdBook with Yarner

[mlange-42]

Wrote core chapters in #41

[mlange-42]

Wrote side topics chapters in #111