Redesign the documentation with the Furo theme

[galjos]

The docs used Sphinx's default alabaster theme, which looks dated and plain. This switches to Furo (clean, modern, zero-config light/dark mode) plus sphinx-copybutton and sphinx-design, and refreshes the landing page.

Theme

• html_theme = "furo", a brand accent color (teal, light + dark variants), a GitHub footer icon, sphinx-copybutton (copy-to-clipboard on every code block), and intersphinx to Python/numpy docs.
• All new deps are added to the docs extra only — no runtime/core impact.

Landing page (index.rst)

Rewritten: PyPI/Python-version/license/build/coverage badges, a verified quick-start snippet (pulled from usage.rst, not invented), and a sphinx-design feature-card grid. The old landing page only described the original DFTB+/xtb feature set — the grid now also covers reactions/redox, transition-state kinetics, and the ORCA/cclib/PySCF QM imports built this session, none of which were mentioned before.

Tables

installation.rst and configuration.rst had hand-written ASCII grid tables that wrapped awkwardly (e.g. tblite-python split mid-word across lines). Converted to list-table directives — cleaner source and more consistent rendering.

Verification

Built locally and visually reviewed every page (landing, API reference, usage, configuration) in both light and dark mode via a local preview server + browser screenshots — not just "the build succeeded." Zero Sphinx warnings (also fixed a pre-existing broken ase intersphinx inventory URL that was about to start warning). Full test suite passes (this PR only touches docs/ and the docs extra in pyproject.toml).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.25%. Comparing base (3e3307a) to head (ccf8897).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #106   +/-   ##
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  Coverage   97.25%   97.25%           
=======================================
  Files          31       31           
  Lines        1895     1895           
=======================================
  Hits         1843     1843           
  Misses         52       52           

Flag	Coverage Δ
unittests	97.25% <ø> (ø)

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