recompiling the sphinx docs fails (neurodamus)

If I try to compile docs for neurodamus, the second time it fails with:

```
no targets are out of date.

Traceback (most recent call last):
File "/Users/katta/OBI/neurodamus/.tox/docs/lib/python3.11/site-packages/sphinx/events.py", line 94, in emit
results.append(listener.handler(self.app, *args))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/katta/OBI/neurodamus/.tox/docs/lib/python3.11/site-packages/sphinx_bluebrain_theme/utils/search_builder.py", line 139, in copy_search_index_json
copy_asset_file(str(file_), output, context=app.builder.globalcontext)
^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'StandaloneHTMLBuilder' object has no attribute 'globalcontext'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/Users/katta/OBI/neurodamus/.tox/docs/lib/python3.11/site-packages/sphinx/cmd/build.py", line 276, in build_main
app.build(args.force_all, filenames)
File "/Users/katta/OBI/neurodamus/.tox/docs/lib/python3.11/site-packages/sphinx/application.py", line 331, in build
self.events.emit('build-finished', None)
File "/Users/katta/OBI/neurodamus/.tox/docs/lib/python3.11/site-packages/sphinx/events.py", line 102, in emit
raise ExtensionError(__("Handler %r for event %r threw an exception") %
sphinx.errors.ExtensionError: Handler for event 'build-finished' threw an exception (exception: 'StandaloneHTMLBuilder' object has no attribute 'globalcontext')

Extension error (sphinx_bluebrain_theme.utils.search_builder):
Handler for event 'build-finished' threw an exception (exception: 'StandaloneHTMLBuilder' object has no attribute 'globalcontext')
```

I suspect there is a problem in `search_builder.py` because it refers to a property that is missing in `StandaloneHTMLBuilder`.