switch GitHub pages to gh-pages branch

- Updates .asf.yaml to use the gh-pages branch as the source
for GitHub pages (published to `https://apache.github.io/couchdb-helm`).
- Require linear history for the `main` branch to ensure
   PRs are up to date prior to merge.
- Prevent force push to `gh-pages` branch.
diff --git a/.asf.yaml b/.asf.yaml
index 3c754c3..e7d691f 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -8,5 +8,6 @@
     main:
       required_status_checks:
         strict: true
-  ghp_branch:  main
-  ghp_path:    /docs
+      required_linear_history: true
+    gh-pages: {}
+  ghp_branch: gh-pages