Update installation docs

[AntoinePrv]

• No longer indicate that micromamba is experimental,
• Add micromamba self-update
• Add conda-forge setup
• Restructure sections a bit
• Change curl ... | bash command to process user input

[JohanMabille]

Nice improvement, only a typo to fix and this is ready to go!

[RobbertDM]

Just FYI:
The previous command stopped working because that didn't have the -L curl option.
The new command with the <() process substitution is not supported in sh. So if you had a #!/bin/sh at the top of some script like we did, change it to #!/bin/bash (will run with interaction) or just keep the old script and add -L to the curl command.