Add exoearth spectroscopy tutorial and update related configurations

[AarynnCarter]

This pull request adds support for a new tutorial notebook called exoearth_spectroscopy_tutorial.ipynb to the documentation. The changes ensure that this new tutorial is included in both the documentation build process and the documentation index.

Documentation updates:

• Updated the list of notebooks to copy in conf.py to include exoearth_spectroscopy_tutorial.ipynb, ensuring it is available in the documentation source.
• Added exoearth_spectroscopy_tutorial to the documentation index in index.md so it appears alongside other tutorials.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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