Fix abstracts for the splits of `overview`

[dbosk]

Currently each session broken out from overview has the old abstract from overview as an abstract. Obviously this is not accurate anymore. The affected sessions are:

• [x] intro
• [x] bootstrapping
• [ ] user-machine
• [ ] something-you-know
• [ ] something-you-have
• [ ] something-you-are
• [ ] machine-user