Fix Links

from https://codebase.helmholtz.cloud/hifis/overall/hifis-technical-documentation/-/jobs/1516783 :

• https://codebase.helmholtz.cloud/hifis/overall/kpi/kpi-plots-ci/-/jobs/artifacts/master/file/plots/pdf/helmholtz-cloud-services-plot_2.pdf?job=plot_general (HTTP_404)
• https://hifis.net/roadmap/ (HTTP_404)
• https://codebase.helmholtz.cloud/hifis/overall/kpi/kpi-plots-ci/-/jobs/artifacts/master/browse/plots_kpi?job=plot_general (HTTP_404)
• http://www.gnuplot.info/docs_5.4/Gnuplot_5_4.pdf (HTTP_409)
• https://codebase.helmholtz.cloud/help/ci/introduction/index.md (HTTP_404)