Support build-time SVG rendering in Maven/Gradle via the CLI

[brunoborges]

Description

#279 asks for a way to render SVG logos to PNG/JPEG at build time, for example PWA icons. The prototype for this is viritin/svg-render-maven-plugin. This PR does it without a new Maven plugin artifact: the build runs the existing CLI (io.brunoborges.jairosvg.cli.Main) in the same JVM, using exec-maven-plugin's java goal with jairosvg added as a plugin dependency. Gradle can do the same with a JavaExec task.

The CLI needed three small changes to work reliably inside a build:

• Missing output parent directories are now created. Before, -o target/classes/META-INF/resources/icons/icon.png failed if the folder did not exist.
• Headless AWT is now the default. Main sets java.awt.headless=true unless the property is already set. This matters because the CLI now runs inside the Maven JVM.
• Invalid arguments are now rejected. Unknown options, options with no value, and non-numeric values throw IllegalArgumentException with a clear message. This way a typo such as --outputwidth fails the build instead of producing an image at the wrong size.
  • Behavior change: unknown flags used to be silently ignored. testUnknownFlagIgnored is replaced by tests for the rejection cases.

Docs:

• New site page build-time-rendering.md, linked from the site menu and cli.md. It covers:
  • a basic Maven example
  • several sizes from one SVG (apple-touch-icon, 192, 512)
  • notes on output format and size
  • a generated-resources + build-helper alternative
  • a Gradle Kotlin DSL example
  • credit to the viritin prototype
• New short README section with the Maven snippet.
• Removed -i, --invert-images from the README CLI table, because the CLI never implemented it.
• bump-version.sh now also updates the version shown in the new page.

Compared with the dedicated plugin, this gives up {size} placeholders, fingerprint-based incremental skipping, and automatic resource registration. In return there is no new artifact to release or maintain.

Validation:

• Ran the recipe against a throwaway sample project using the locally installed snapshot. mvn package put a 512x512 PNG and a 192x96 JPEG into the jar. Changing an argument to the typo --outputwidth failed the build with Unknown option.
• ./mvnw clean verify site: 1272 tests, 0 failures, and the new page was generated. The site step also logged a javadoc report error, but the build still succeeded.

Fixes: #279

Checklist

• Tests pass: ./mvnw clean verify (BUILD SUCCESS, 1272 tests, 0 failures)
• Code formatted: ./mvnw spotless:apply
• Documentation updated (if applicable)
• No breaking changes to public API (library API unchanged; the CLI now rejects unknown flags it used to ignore)