Add Light and Dark Mode Logos to Documentation

[harryswift01]

Summary

Update the prepmd documentation to support separate light and dark mode logos, keeping the branding consistent with other CCPBioSim repositories.

Changes

Documentation logos:

• Added light and dark variants of the prepmd logo to the Sphinx documentation.
• Updated the Sphinx configuration to use the appropriate logo for light and dark themes.
• Updated the README to reflect the documentation branding changes.

Impact

• The prepmd documentation now displays the appropriate logo based on the selected Furo theme.
• No changes to the package functionality or API.