Update gtk-dev-guide.md: Modernize references and add GTK4 documentation (#5)

Update gtk-dev-guide.md: Replace Bugzilla and wiki references with GitHub
Add build verification section to gtk-dev-guide.md
Update gtk-dev-guide.md: Use local images instead of wiki.eclipse.org links

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: laeubi <[email protected]>
Co-authored-by: Christoph Läubrich <[email protected]>

Author: Copilot