docs: improve persistence pages with examples and state store

[alexeyzimarev]

Summary

• Event store page: added usage examples covering appending events, multi-stream append, reading events, stream management (StreamExists, TruncateStream, DeleteStream), and aggregate load/store via extension methods
• State store page (new): documents LoadState extension methods on IEventReader — loading state by stream name, loading by typed identity with StreamNameMap, the FoldedEventStream<TState> return type, and how it underpins both command service variants
• Event streams page (renamed from "Aggregate stream"): rewritten to be state-centric — covers StreamName.For<T> (unconstrained), StreamName.ForState<TState>, uses Id instead of deprecated AggregateId, notes applicability to both aggregate-based and functional command services
• Fixed relative links in next/ version that incorrectly escaped the next/ directory

All changes applied to both v0.16 (stable) and next (preview).

Test plan

• pnpm build passes (140 pages)
• Spot-check rendered pages in both version tabs

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