Putty connecting advice doesn't cover use of ssh keys

[james-grant1]

Add link/details to use of puttygen if necessary to generate
Add link/details to use of pageant to store keys to ease access via bastion
Add link/details for adding key to putty connection

Also check that advice for Linux includes safe ssh use e.g. point to: https://arc-lessons.github.io/security/00_schedule.html which also contains advice for putty via references as well.