docs: Refactor docs page and add automatic upload

.gitlab-ci.yml

docs/.gitlab-ci.yaml

docs/Makefile

docs/src/SUMMARY.md

docs/src/about.md

docs/src/charts/hedgedoc.md

docs/src/charts/mastodon.md

docs/src/charts/nut-exporter.md

docs/src/components/flux.md

docs/src/components/k8up.md

docs/src/components/monitoring.md

docs/src/concepts/README.md

docs/src/concepts/gitops.md

docs/src/concepts/sre.md

docs/src/hardware/README.md

docs/src/hardware/nas.md

docs/src/hardware/ups.md

docs/src/images/demo.md

docs/src/images/demo.md

docs/src/operating_system.md