CONTRIB: move src/wurfl.c and contrib/wurfl to addons/wurfl

Both the source file and the dummy library are now at the same place.
Maybe the build howto could be moved there as well to make things even
cleaner.

The Makefile, MAINTAINERS, doc, and vtest matrix were updated.
diff --git a/.github/workflows/vtest.yml b/.github/workflows/vtest.yml
index 6d42db8..fc6eb55 100644
--- a/.github/workflows/vtest.yml
+++ b/.github/workflows/vtest.yml
@@ -78,7 +78,7 @@
       run: env ${{ matrix.ssl }} scripts/build-ssl.sh
     - name: Build WURFL
       if: ${{ contains(matrix.FLAGS, 'USE_WURFL=1') }}
-      run: make -C contrib/wurfl
+      run: make -C addons/wurfl/dummy
     - name: Compile HAProxy with ${{ matrix.CC }}
       run: |
         make -j$(nproc) all \