Fix to publish docs on pushes to main branch (#722)

## Summary
- I believe we need to publish docs on pushes to `main` branch, not
`gh-pages` (`mkdocs gh-deploy --force` itself pushes to `gh-pages`)
- To resolve #641

## Testing Done
None

- Hardware Type: <BLANK>
- [ ] run `make test` to ensure correctness
- [ ] run `make checkstyle` to ensure code style
- [ ] run `make test-convergence` to ensure convergence

Author: shimizust

.github/workflows/docs.yml

@@ -2,7 +2,7 @@ name: Publish documentation
 on:
   push:
     branches:
-      - gh-pages
+      - main
 permissions:
   contents: write
 jobs: