Update the extract_celix_docs.sh script to also copy images

Often the Celix Markdown files include images. However, it currently only copies over Markdown files and for the 2.3.0 release I manually copied the applicable PNG files.

It would be better if the [extract_celix_docs.sh](https://github.com/apache/celix-site/blob/master/extract_celix_docs.sh) script is able to copy images.