docs.rs: building fewer targets by default

Starting 2026-05-01, docs.rs will change its default build behavior: instead of building documentation for five targets, it will only build for the default target (x86_64-unknown-linux-gnu) unless additional targets are explicitly specified in Cargo.toml metadata. Crates needing multi-target docs must define a full targets list in their docs.rs metadata. This change affects new releases and rebuilds of old releases, reduces build times, and saves resources.