Stable version as default in docs

[leonlan]

Is your feature request related to a problem? Please describe

Most users install PyVRP through PyPI and so they use the stable version. However, www.pyvrp.org shows the documentation for the latest development version by default, possibly confusing users when they find features in the docs that aren't released yet.

Describe the solution you'd like

• www.pyvrp.org should default to the stable version of PyVRP.
• www.pyvrp.org/dev should default to the latest development version of PyVRP.