Update Git, Conda and Snakemake tutorials

[johnne]

This PR contains minor fixes and updates for the Git and Conda tutorials and a relatively large rewrite of the Snakemake tutorial.

For the Snakemake tutorial the most important changes are:

• Require Snakemake version 8 or higher
• Modification of commands to be compatible with Snakemake version 8 or higher
• Removal of the generate_rulegraph rule
• Condensed and clearer descriptions throughout