Document extending ConcurrencySystem with custom load signals

[janbuchar]

• added in feat: Modularize SystemStatus to allow custom backpressure mechanisms #3529
• currently, this is an undocumented feature (and not part of a stable release)
• refactor!: Split off ConcurrencySystem from AutoscaledPool #3917 moved signals from systemStatusOptions.loadSignals to ConcurrencySystemOptions.loadSignals and made the whole extension point public - LoadSignal, LoadSnapshot, LoadSignalStartContext, SnapshotStore and the four built-in signal classes (MemoryLoadSignal, EventLoopLoadSignal, CpuLoadSignal, ClientLoadSignal), so the API reference side of this is covered
• what's missing is guide-level docs: scaling_crawlers.mdx walks through every scaling option of ConcurrencySystem but never mentions loadSignals, and the only prose about writing (or wrapping) a signal sits in the v4 upgrading guide, which new users don't read - needs a section plus a runnable scaling_crawlers_customLoadSignal.ts example