docs/conf.py: Use modern intersphinx_mapping format

[EwoutH]

With Sphinx 8.0 they removed the old intersphinx_mapping format we were still using, which was introduced in Sphinx 0.5. This means an 10 year old line of code got our docs failing.

This commit updates the docs/conf.py file to use the new intersphinx_mapping format, as described here.

Resolves #2205.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 -0.1% [-0.5%, +0.3%]	🔵 -0.3% [-0.6%, -0.1%]
Schelling	large	🔵 +0.0% [-0.6%, +0.5%]	🔵 -1.2% [-3.4%, +1.1%]
WolfSheep	small	🔵 +0.6% [-0.6%, +1.9%]	🔵 +0.8% [+0.4%, +1.2%]
WolfSheep	large	🔵 -0.8% [-1.3%, -0.2%]	🟢 -4.5% [-5.8%, -3.2%]
BoidFlockers	small	🔵 +1.1% [+0.5%, +1.7%]	🔵 -1.8% [-2.6%, -1.1%]
BoidFlockers	large	🔵 +1.8% [+1.3%, +2.3%]	🔵 +0.4% [-0.2%, +1.0%]