BUG: Only run jupyter lite on RTD HTML build

To address:

```
translation:copy
-- build:contents:copy:Hello3DWorld.ipynb
.  build:lite:patch:_build/html/_static/jupyter-lite.json
TaskError - taskid:build:lite:patch:_build/html/_static/jupyter-lite.json
PythonAction Error
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/itkwidgets/envs/latest/lib/python3.7/site-packages/doit/action.py", line 437, in execute
    returned_value = self.py_callable(*self.args, **kwargs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/itkwidgets/envs/latest/lib/python3.7/site-packages/jupyterlite/addons/base.py", line 223, in merge_one_jupyterlite
    out_path.write_text(json.dumps(config, **JSON_FMT), **UTF8)
  File "/home/docs/.pyenv/versions/3.7.9/lib/python3.7/pathlib.py", line 1240, in write_text
    with self.open(mode='w', encoding=encoding, errors=errors) as f:
  File "/home/docs/.pyenv/versions/3.7.9/lib/python3.7/pathlib.py", line 1208, in open
    opener=self._opener)
  File "/home/docs/.pyenv/versions/3.7.9/lib/python3.7/pathlib.py", line 1063, in _opener
    return self._accessor.open(self, flags, mode)
FileNotFoundError: [Errno 2] No such file or directory: '/home/docs/checkouts/readthedocs.org/user_builds/itkwidgets/checkouts/latest/docs/_output/_build/html/_static/jupyter-lite.json'
```

Author: thewtex

docs/conf.py

@@ -76,5 +76,5 @@ def jupyterlite_build(app: Sphinx, error):
 def setup(app):
     # For local builds, run jupyter lite build manually
     # $ jupyter lite serve --config ./jupyterlite_config.json
-    if RTD:
+    if RTD and app.builder.format == "html":
         app.connect("config-inited", jupyterlite_build)