Add static homepage, Dockerfile (nginx), and CI build/push workflow

- index.html: clean static homepage (name + intro, links), light/dark mode
- Dockerfile: nginx:1.27-alpine serves the page on :80 with a healthcheck
- .gitea/workflows/build.yml: on push to main, build and push the image to
  git.ffaerber.duckdns.org/ffaerber/homepage (tags :latest and :sha)

Dockerfile

@@ -0,0 +1,11 @@
+FROM nginx:1.27-alpine
+
+LABEL org.opencontainers.image.title="ffaerber/homepage" \
+      org.opencontainers.image.source="https://git.ffaerber.duckdns.org/ffaerber/homepage"
+
+COPY index.html /usr/share/nginx/html/index.html
+
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget -qO- http://127.0.0.1/ >/dev/null 2>&1 || exit 1