Update to v0.7.1

MANTIS 0.7.1 introduces official Sphinx and Read the Docs documentation integration alongside comprehensive docstring cleanups across the codebase. This release reorganizes the tutorial suite into two modular guides (Tutorial_1 for fundamentals and setup, and Tutorial_2 for advanced multi-element isotopes and carbonate chemistry), adds $^{13}\text{C}$ carbon isotope tracking to the calcite dissolution benchmark model, updates CI actions to v6, and includes full regression benchmark datasets and tests for the Luff & Wallmann (2003) model.