Support asciidoc files

Investigate if gardener can be used to generate images in asciidoc. If so, modify code to pick up asciidoc.

Start by examining how gardener currently handles Markdown image tags and PlantUML diagram sources, then investigate whether the same image generation can work with Asciidoc files. Confirm the supported Asciidoc behavior and finish when the utility picks up those files and generates the expected images.

Written by the indexing model from the issue text.