Provide bazel-maintained fallback font for PlantUML-parsing

[Ehrja]

Fundamentally driven via GH-copilot: Claude Sonnet 5

Problem:
When Sphinx documentation builds render PlantUML diagrams inside a minimal container or toolchain that has no native fontconfig library and no fonts installed, PlantUML crashes instead of rendering:

Exception in thread "main" java.lang.InternalError: java.lang.reflect.InvocationTargetException
	at java.desktop/sun.font.FontManagerFactory$1.run(FontManagerFactory.java:86)
	...
Caused by: java.lang.RuntimeException: Fontconfig head is null, check your fonts or fonts configuration
	at java.desktop/sun.awt.FontConfiguration.getVersion(FontConfiguration.java:1271)
	at java.desktop/sun.awt.FontConfiguration.readFontConfigFile(FontConfiguration.java:225)
	at java.desktop/sun.awt.FontConfiguration.init(FontConfiguration.java:107)
	at java.desktop/sun.awt.X11FontManager.createFontConfiguration(X11FontManager.java:719)
	...

Solution:
Added a fallback mechanism of a bazel-maintained font in case no-system-font is available.

The proposed font was selected based on its license being compatible with https://www.eclipse.org/legal/licenses/