Update doc building to publish release asset

The automatic doc building in GH Actions will publish a release asset,
not a workflow artifact. This is necessary in order for the built docs
to be persistent (workflow artifacts expire).

The downside is that if docs.globus.org references a build artifact,
we might need to "backport" any doc fixes into an old release.
However, there should be no harm in doing so, and we aren't promising
externally that "released" docs are immutable.

Author: sirosen

.github/workflows/docs.yaml

@@ -1,8 +1,8 @@
+# build and release new docs as a release asset
 name: Build Docs
 on:
-  push:
-    branches:
-      - autobuild-doc
+  release:
+    types: [created]
 jobs:
   doc:
     name: Build Docs
@@ -21,16 +21,14 @@ jobs:
           ./reference/_generate.py
           # bundle as tarball without the _generate.py script
           tar --exclude "*.py" -czf cli-reference.tar.gz reference/
-      # upload as an artifact -- to download, use the GitHub APIs
-      # you can find this workflow's ID by listing workflows using:
-      #   https://docs.github.com/en/rest/reference/actions#list-repository-workflows
-      # then find the last run for a branch:
-      #   https://docs.github.com/en/rest/reference/actions#list-workflow-runs-for-a-repository
-      #
-      # and list artifacts for a run:
-      #   https://docs.github.com/en/rest/reference/actions#list-workflow-run-artifacts
+      # upload as a release asset
       - name: Upload Autodoc
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-release-asset@v1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         with:
-          name: cli-reference
-          path: ./cli-reference.tar.gz
+          # upload URL is pulled from the release event
+          upload_url: ${{ github.event.release.upload_url }}
+          asset_path: ./cli-reference.tar.gz
+          asset_name: cli-reference.tar.gz
+          asset_content_type: application/gzip