ci: github: Only build docs for zephyrproject-rtos GH org

As documentation is now built using one of our self-hosted runners,
have the GitHub action only run for the upstream repo, not for potential
forks.

(cherry picked from commit b1c2075)

Original-Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
GitOrigin-RevId: b1c2075
Change-Id: Ic2f7d99d823b7d016a3ddcda0e58fc1e50967db4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/4964726
Tested-by: Al Semjonovs <asemjonovs@google.com>
Commit-Queue: Al Semjonovs <asemjonovs@google.com>
Reviewed-by: Fabio Baltieri <fabiobaltieri@google.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>

.github/workflows/doc-build.yml

@@ -35,6 +35,7 @@ env:
 jobs:
   doc-build-html:
     name: "Documentation Build (HTML)"
+    if: github.repository_owner == 'zephyrproject-rtos'
     runs-on: zephyr-runner-linux-x64-4xlarge
     timeout-minutes: 45
     concurrency:
@@ -130,7 +131,9 @@ jobs:
 
   doc-build-pdf:
     name: "Documentation Build (PDF)"
-    if: github.event_name != 'pull_request'
+    if: |
+      github.event_name != 'pull_request' &&
+      github.repository_owner == 'zephyrproject-rtos'
     runs-on: zephyr-runner-linux-x64-4xlarge
     container: texlive/texlive:latest
     timeout-minutes: 60