Only include PNG files in reference manual
Only include PNG files in reference manual

The `images` folder contains some files used to create the images, such
as OmniGraffle and SVG.

This commit modifies the `include` pattern to ensure that only `*.png`
files are copied from the source folder to the published reference
manual.

spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework