Add CI for docs

ci/build.linux.gitlab-ci.yml

@@ -0,0 +1,30 @@
+# Base configuration for linux cross-compilation.
+.build:linux:cross:
+  extends: .common:linux:cross
+  stage: build
+  variables:
+    EIGEN_CI_BUILD_TARGET: buildtests
+  script:
+    - . ci/scripts/build.linux.script.sh
+  tags:
+    - saas-linux-2xlarge-amd64
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "schedule" && $CI_PROJECT_NAMESPACE == "libeigen"
+    - if: $CI_PIPELINE_SOURCE == "web" && $CI_PROJECT_NAMESPACE == "libeigen"
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event" && $CI_PROJECT_NAMESPACE == "libeigen" && $CI_MERGE_REQUEST_LABELS =~ "/all-tests/"
+  cache:
+    key: "$CI_JOB_NAME_SLUG-$CI_COMMIT_REF_SLUG-BUILD"
+    paths:
+      - ${EIGEN_CI_BUILDDIR}/
+
+build:linux:docs:
+  extends: .build:linux:cross
+  variables:
+    EIGEN_CI_TARGET_ARCH: any
+    EIGEN_CI_BUILD_TARGET: doc
+    EIGEN_CI_INSTALL: ca-certificates clang flex python3 bison graphviz
+    EIGEN_CI_C_COMPILER: clang
+    EIGEN_CI_CXX_COMPILER: clang++
+    EIGEN_CI_BEFORE_SCRIPT: ". ci/scripts/build_and_install_doxygen.sh Release_1_13_2"
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "push" && $CI_PROJECT_NAMESPACE == "libeigen"