hexdump(1): Add EXAMPLES section

• Add examples showing the use of -f, -C, -s, -n
• Rework the two already present examples that were *format* examples
• Remove .Tn suggested by mandoc(1)
• Remove reference to gdb(1) since it is not present in current

Approved by: manpages (gbe)
Differential Revision: https://reviews.freebsd.org/D25406