Add site_url (#2130)

### Changes

- Tells MkDocs where the site will be published at allowing it to
generate a sitemap

---------

Co-authored-by: emma <hi@emma.cafe>

.github/workflows/docs-publish.yaml

@@ -27,7 +27,8 @@ jobs:
           DOCS_SOURCE_URL: https://app.browsertrix.com
           ENABLE_ANALYTICS: true
 
-      - name: Build Docs
+      - name: Build & Deploy Docs
         run: cd frontend/docs; mkdocs gh-deploy --force
         env:
+          SITE_URL: https://docs.browsertrix.com
           ENABLE_ANALYTICS: true

frontend/docs/mkdocs.yml

@@ -1,4 +1,5 @@
 site_name: Browsertrix Docs
+site_url: !ENV SITE_URL
 repo_url: https://github.com/webrecorder/browsertrix-cloud/
 repo_name: Browsertrix
 edit_uri: edit/main/frontend/docs/docs/