Add OSA chat widget to documentation

[neuromechanist]

Summary

• Adds the Open Science Assistant (OSA) chat widget to the MNE-Python documentation
• The widget provides an AI assistant specialized in MNE-Python that can help users with MEG, EEG, and neurophysiological data analysis questions
• Loads the widget script from the OSA CDN and configures it with 3 suggested questions

Details

The widget is a lightweight floating chat button that appears on all documentation pages. It:

• Uses the mne community configuration from OSA
• Shows 3 suggested starter questions relevant to MNE-Python users
• Supports page context awareness (sends current page URL to help provide contextually relevant answers)
• Is non-intrusive; users can dismiss or ignore it

The widget is served from demo.osc.earth and configured via the communityId: 'mne' setting, which auto-configures the API endpoint, title, theme color, and initial greeting.

Test plan

• CORS origins for mne.tools and *.mne.tools are enabled on the OSA backend
• Verify widget loads on local Sphinx build (make html)
• Verify chat responses work end-to-end
• Check widget appearance on mobile viewports

[welcome]

Hello! 👋 Thanks for opening your first pull request here! ❤️ We will try to get back to you soon. 🚴

[neuromechanist]

Hi maintainers! Could someone approve the CircleCI pipeline? It requires maintainer approval for first-time contributors from forks.

Also, if you'd like to customize the widget further (e.g., adding your own logo), you can pass additional options to OSAChatWidget.setConfig(). For example:

OSAChatWidget.setConfig({
  communityId: 'mne',
  logo: 'https://mne.tools/stable/_static/mne_logo.svg',
  suggestedQuestions: [...]
});

Full configuration reference: https://docs.osc.earth/osa/deployment/widget/#full-configuration