Fix GitHub ref variable for pushing docs to main branch (#343)

According to the
[docs](https://docs.github.com/en/actions/learn-github-actions/contexts#github-context):

> The `head_ref` or source branch of the pull request in a workflow run.
This property is only available when the event that triggers a workflow
run is either `pull_request` or `pull_request_target`.

So we remove the `head_ref` and replace it with `ref`:

> The fully-formed ref of the branch or tag that triggered the workflow
run. For workflows triggered by push, this is the branch or tag ref that
was pushed. For workflows triggered by `pull_request`, this is the pull
request merge branch. For workflows triggered by release, this is the
release tag created. For other triggers, this is the branch or tag ref
that triggered the workflow run. This is only set if a branch or tag is
available for the event type. The ref given is fully-formed, meaning
that for branches the format is `refs/heads/<branch_name>`, for pull
requests it is `refs/pull/<pr_number>/merge`, and for tags it is
`refs/tags/<tag_name>`. For example, `refs/heads/feature-branch-1`.

Second option: This can be also replaced with `ref_name`:

> The short ref name of the branch or tag that triggered the workflow
run. This value matches the branch or tag name shown on GitHub. For
example, `feature-branch-1`.

.github/workflows/ci.yaml

@@ -86,12 +86,10 @@ jobs:
 
   publish-docs-from-main:
     runs-on: ubuntu-22.04
-    if: ${{ github.head_ref == 'refs/heads/main' }}
+    if: ${{ github.ref == 'refs/heads/main' }}
     needs: [test]
     steps:
       - uses: actions/checkout@v3
-        with:
-          ref: ${{ github.head_ref }}
 
       - name: Publish docs from main branch
         uses: ./.github/actions/update-docs