Docs: Add bullet number images removed in 2.10

Docs: Add bullet number images removed in 2.10

These number images are used by the code callout feature of AsciiDoc,
used by the new Receipt Template Editor chapter (and possibly other
docs). I had renamed them in 2015, supposedly to prevent an ePub build
error, but that broke the code callout images. And since I cannot
replicate the ePub error now, I am renaming them back to their original
state, with sincere apologies to code callout readers everywhere.

Signed-off-by: Remington Steed <rjs7@calvin.edu>