Run docs-ci only on the current default branch

This avoids that outdated docs are published by pushing to older
branches.

.github/workflows/docs-ci.yml

@@ -1,24 +1,20 @@
-# This workflow will build and deploy docs to GitHub Pages whenever something is pushed to 0.8.x
+# This workflow will build and deploy docs to GitHub Pages whenever something is pushed to the default branch
 
 name: Docs
 
-on:
-  push:
-    branches:
-      - 0.8.x
+on: push # but only to default branch, see if below
 
 permissions:
   contents: write
 
-# Allow one concurrent deployment
-concurrency:
-  group: pages
-  cancel-in-progress: true
-
 jobs:
   docs:
     name: Build and deploy docs
     runs-on: ubuntu-latest
+    if: github.ref == format('refs/heads/{0}', github.event.repository.default_branch)
+    concurrency: # Allow one concurrent deployment
+      group: pages
+      cancel-in-progress: true
     env:
       GITHUB_TAG_NAME: ${{ github.event.release.tag_name }}
       GITHUB_BRANCH_NAME: ${{ github.ref }}