handbook/jails: Small fixes to the Linux jail section
- Add missing containers/ path in one instance
- Move the jail.conf creation to the beginning of this section (slightly modified from the original patch), because it fits better there instead of letting the reader know about it when starting the jail
- Use sh as the shell executed when running jexec to ensure it is a jail that is present in Linux
- Rephrase a sentence to avoid a comma and instead use "and" to connect the two packages to install (also modified from the original submission)
Event: Halifax Hackathon 202606
PR: 295777
Differential Revision: https://reviews.freebsd.org/D57771