re #15 docs: Cache package guide

[tonydspaniard]

Summary

Cache package documentation, per #15. Single new file: docs/packages/cache.md (~656 lines). Follows the structure established by #16 (Cookie): introduction, installation, quick start, concepts, usage, configuration, testing, extending, recipes, related packages, migration notes, limitations.

Covers PSR-6 and PSR-16 surfaces, all four backends (Filesystem / Memcached / Redis / Predis), namespace + lifetime semantics, deferred saves, the Configuration classes and their env vars.

Source observations

• FilesystemCacheItemStorage stores zero-lifespan items with a 31,557,600-second (one-year) expiry — a deliberate workaround for the mtime-based hasItem(). Documented in the lifetime section.
• CacheItemPool detects PredisCacheItemStorage specifically and calls useNamespace() directly on it — the pool skips its own prefix for Predis. Documented as a double-prefix risk.

Spun out as a separate issue

• cache: RedisCacheItemStorageConfiguration wires PredisCacheItemStorage; ext-redis lacks wiring #50 — RedisCacheItemStorageConfiguration wires PredisCacheItemStorage (despite the name); the ext-redis variant ships without a Configuration class.

Test plan

• Render the page on GitHub, skim for layout
• Cross-links to sibling docs (./container.md, etc.) will resolve as those PRs merge
• Pure docs additive — no code touched