ci(docs): use GitHub App client ID for docs publishing

Upgrade actions/create-github-app-token to v3 and authenticate with
client-id instead of the deprecated app-id input. The client ID is read
from the DOCS_PUBLISH_APP_CLIENT_ID repository variable.

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>
Assisted-by: GitHub Copilot: Claude Haiku 5.5

Author: fpistm

.github/workflows/publish-docs.yml

@@ -37,10 +37,10 @@ jobs:
         run: |
           git config user.name "github-actions[bot]"
           git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
-      - uses: actions/create-github-app-token@v1
+      - uses: actions/create-github-app-token@v3
         id: pages-token
         with:
-          app-id: ${{ secrets.DOCS_PUBLISH_APP_ID }}
+          client-id: ${{ vars.DOCS_PUBLISH_APP_CLIENT_ID }}
           private-key: ${{ secrets.DOCS_PUBLISH_APP_PRIVATE_KEY }}
           owner: stm32duino
           repositories: stm32duino.github.io