ci: add Codeberg CI (Woodpecker + Forgejo) and document CI/CD topology

Add a lightweight test-fast pipeline for the public Codeberg mirror so it
shows a build status, defined for both CI systems Codeberg offers:
- .woodpecker.yml            (Woodpecker, debian:bookworm)
- .forgejo/workflows/        (Forgejo Actions, node:20-bookworm, runner label docker)

Both run the pure-math suite only (Eigen vendored, GTest via FetchContent;
no CGAL/Boost), using public images and the docker runner label since the
private eulernest ci-cpp image and Pi runner are unreachable from Codeberg.

Docs:
- new doc/architecture/ci-cd.md: two-forge topology, runners, trigger
  keywords, mirror mechanism.
- fix stale "codeberg/main ... no CI" claim in contributing.md and bring
  the CI table up to date.
- link the new doc from the README doc index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.woodpecker.yml

@@ -0,0 +1,31 @@
+# Woodpecker CI — Codeberg (https://ci.codeberg.org)
+#
+# Runs on every push / PR.  Mirrors the "test-fast" job of the
+# eulernest Gitea pipeline (.gitea/workflows/cpp-tests.yml), but uses a
+# plain public Debian image instead of the private ci-cpp registry image,
+# because Codeberg's shared runners cannot pull git.eulernest.eu.
+#
+# Pure-math test suite only (Clausen, ImLi2, hyper-ideal geometry):
+#   * Eigen is vendored      → code/deps/eigen-3.4.0
+#   * GoogleTest via FetchContent (network at configure time)
+#   * No CGAL / Boost / Wayland needed  → fast, headless, < 2 min
+#
+# A failing build or any failing ctest makes the Codeberg CI badge red.
+
+when:
+  - event: [push, pull_request]
+
+steps:
+  test-fast:
+    image: debian:bookworm
+    commands:
+      - apt-get update -qq
+      - >-
+        apt-get install -y --no-install-recommends
+        cmake g++ make git ca-certificates
+      - cmake -S code -B build -DCMAKE_BUILD_TYPE=Release
+      - cmake --build build --target conformallab_tests -j$(nproc)
+      - >-
+        ctest --test-dir build
+        --output-on-failure
+        --output-junit test-results.xml