Update intro for latest Nu version

[rgwood]

The book intro shows a lot of examples, and they had not been updated since engine-q. I reran the examples on v0.59.1 and updated the intro accordingly.

Notes

• I intentionally kept things largely the same as before - I ran the obviously-macOS examples on macOS, and the obviously-Linux examples on Linux. And I tweaked the results so the username + machine name are the same.
• It's a bit weird that virtual memory usage is ~32GB for every process on macOS. Suspect it's a bug related to this and this. It's arguably not ideal to show a bug in the intro docs; if people care about this I could run the examples on another OS?
• The date now example isn't quite accurate because of this bug. I left a note there to fix up the docs when the bug is fixed.
• The syntax coloring's a bit off because the entire snippet is interpreted as a bash script. That should probably be changed eventually but it's no worse than before:

[fdncred]

Looks good! Thanks!