ci: docs: build doxygen documentation in a matrix including latest version

Add a matrix to the docs workflow to build the documentation with latest
Doxygen version too.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

Author: kartben

.github/workflows/docs.yml

@@ -5,18 +5,19 @@ name: Documentation
 
 on: [push, pull_request]
 
-env:
-  DOXYGEN_VERSION: 1.9.6
-
 jobs:
   build:
     runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        doxygen-version: [1.9.6, 1.14.0]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
       - name: Install dependencies
         run: |
+          DOXYGEN_VERSION="${{ matrix.doxygen-version }}"
           wget --no-verbose "https://github.com/doxygen/doxygen/releases/download/Release_${DOXYGEN_VERSION//./_}/doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz"
           tar xf doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz
           echo "${PWD}/doxygen-${DOXYGEN_VERSION}/bin" >> $GITHUB_PATH