🎁 Add ability to find pages split from a PDF

[jeremyf]

This commit leverages the conventions established in the DerivativeRodeo around where we're writing split pages and their derivatives.

The inline comments that are written/amended with this PR should be read closely for clarity and intention.

Related to:

• https://github.com/scientist-softserv/iiif_print/issues/251
• https://github.com/scientist-softserv/derivative_rodeo/pull/48