ENH: Add JupyterLite CI/CD infrastructure

[natinew77-creator]

Tracking Issue: #13929

What does this implement/fix?

• Integrates JupyterLite directly into the MNE-Python documentation build using jupyterlite-sphinx.
• Replaces the standalone GitHub Actions workflow (jupyterlite.yml) by hooking into the existing Sphinx-Gallery pipeline, meaning the JupyterLite environment is now built natively alongside the rest of the documentation.
• Configures Sphinx-Gallery to automatically generate "Open as a notebook" interactive buttons for all tutorials and examples.
• Implements a notebook_modification_function in conf.py to automatically inject %pip install -q mne into the generated notebooks, ensuring the browser environment automatically resolves MNE-Python dependencies without requiring manual user input.
• This represents the completion of the first major GSoC milestone to enable interactive "Try in Browser" documentation.

Additional information

• Because this is now integrated directly into Sphinx, CircleCI will automatically build the JupyterLite assets and provide a live preview link in the CI checks.
• Currently, this builds with the default pyodide kernel. In the upcoming weeks, I will explore switching to the xeus-python kernel to better handle complex MNE-Python dependencies if needed.