Documentation improvements #159

Borrowed largely from recent work on PyVRP (see also PyVRP/PyVRP#273):

• Add logos/icons for ALNS.
• Render notebooks for documentation. See Run example notebooks in CI #152 .
• Update notebook phrasing to allow for some variation in results (because they get re-ran every time the docs build).
• Switch documentation theme to sphinx-immaterial.
• Add a page about tuning to the docs.