ci: doc: upgrade to Doxygen 1.9.1

Upgrade to Doxygen version 1.9.1. Package is downloaded directly from
the Doxygen official site, since it is not yet part of the latest Ubuntu
LTS. Note that libclang1-9 and libclang-cpp9 are runtime dependencies
required by Doxygen.

NOTE: Documentation can still be built with older Doxygen versions (e.g.
1.8.17 shipped with latest Ubuntu LTS), however, the template used in
upcoming patches claims to work better with Doxygen 1.9.1 or 1.9.2.
Using 1.9.1 as theme v1.6.0 has some issues on mobile view when using
Doxygen 1.9.2, see
https://github.com/jothepro/doxygen-awesome-css/issues/47.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

.github/workflows/doc-build.yml

@@ -30,6 +30,7 @@ env:
   # The latest CMake available directly with apt is 3.18, but we need >=3.20
   # so we fetch that through pip.
   CMAKE_VERSION: 3.20.5
+  DOXYGEN_VERSION: 1.9.1
 
 jobs:
   doc-build-html:
@@ -42,7 +43,10 @@ jobs:
 
     - name: install-pkgs
       run: |
-        sudo apt-get install -y ninja-build doxygen graphviz
+        sudo apt-get install -y ninja-build graphviz libclang1-9 libclang-cpp9
+        wget -q https://www.doxygen.nl/files/doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz
+        tar xf doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz
+        echo "${PWD}/doxygen-${DOXYGEN_VERSION}/bin" >> $GITHUB_PATH
 
     - name: cache-pip
       uses: actions/cache@v1