feat: Explicitly target Bash and ZSH

[dwmkerr]

Need to make it clear in the intro, front cover and back cover that this is for both Bash and ZSH (as ZSH is the default on MacOS and I believe also one or two Linux distros).

Then in any place where the functionality will differ, use a box to call out the differences. But make it clear that the demos work the same for both shells unless otherwise stated.