Improve the documentation for the page expiry feature

[bravegrape]

The current docs for the page expiry feature are confusing and hard to follow.

The docs should clearly explain:

• what the page expiry feature is
• what are the options to control this feature
• how it relates to the Daniel the Manual Spaniel integration
• the screenshot examples but making sure the screenshots don't break the flow of the page

[jonathanglassman]

Will be covered by https://github.com/alphagov/tdt-documentation/tree/overhaul

[m-green]

Docs overhaul is complete so I think this is done now.