[STAR-3148] docs: Update vacation checklist to point to Listaflow.

This PR links handbook to the vacation checklist. It also removes many of the to-dos which are covered explicitly in the vacation checklist so that there's no chance the two get out of sync.

JIRA tickets: https://tasks.opencraft.com/browse/STAR-3148

Author notes and concerns:

There's some discernment here on which items on the vacations page were best to move entirely to the checklist, and which should still be in the handbook. This means I haven't entirely removed all redundant information between the two, but I have removed most, and kept in the handbook those things that seem most oriented toward the more meta aspects of how vacations are coordinated.