Write user doc for new genetics

[TheoPannetier]

• [x] Changes in Version 3.0
• [x] Genetics 2.0
• [x] Genetics 3.0
• [x] Batch mode section
• [x] Outputs - 2.0 vs 3.0
• [x] Round of updates for other sections
• [x] Exercise 4
• [x] Exercise 5
• [x] Documentation of parameter files

[TheoPannetier]

All done for now, need a review