Build Antora docs as part of CI check job

This commit adds the "antora" task to the check-project script in order
to check for documentation failures when pushing to the current branch.
The actual documentation is build is performed on the docs-build branch.

Closes gh-30528

ci/scripts/check-project.sh

@@ -5,5 +5,5 @@ source $(dirname $0)/common.sh
 
 pushd git-repo > /dev/null
 ./gradlew -Dorg.gradle.internal.launcher.welcomeMessageEnabled=false -Porg.gradle.java.installations.fromEnv=JDK17,JDK20 \
-  -PmainToolchain=${MAIN_TOOLCHAIN} -PtestToolchain=${TEST_TOOLCHAIN} --no-daemon --max-workers=4 check
+  -PmainToolchain=${MAIN_TOOLCHAIN} -PtestToolchain=${TEST_TOOLCHAIN} --no-daemon --max-workers=4 check antora
 popd > /dev/null