Observed an issue when attempting to process tinyxml documentation. After generating XML documentation from tinyxml, processing it through breathe would result in the following warning:

sphinx.errors.SphinxWarning: ...\tinyxml.rst::image file not readable: xml/tinyxml/https:/github.com/leethomason/tinyxml2/actions/workflows/test.yml/badge.svg

The project prefix xml/tinyxml/ had been injected into the URI of the image. This commit tries to help mitigate using an absolute path building for external images by checking if the URI defines a URL schema (using implementation from Sphinx's utility class). If a URL schema is detected, assume the source is an external image and just passthrough the value to the generate image node's uri value. Otherwise, default to building a Sphinx project's absolute path to the image.

• Crudely tested with tinyxml (includes external images) and oglft's documentation (with internal images).
• Sphinx's url_re call should be available in oldest supported Sphinx revision listed (v2.3.1).