docs: document the application cache and its backends

[anoziere]

Summary

• New page getting-started/application-cache.md, and its copy under versioned_docs/version-3.0: what the application cache is and how it differs from the container cache and from an HTTP proxy cache, how to point it at Redis, Valkey or Memcached with THELIA_CACHE_DSN, what each pool holds and what emptying it costs, how installations are kept apart with THELIA_CACHE_PREFIX_SEED, the eviction policy to choose in production and what it does to API sessions, how to check that it works, and a troubleshooting list.
• getting-started/configuration.md: the Caching section now separates the container cache from the application cache and links to the new page, instead of stating that the cache is in var/cache/ and configured in config/packages/cache.yaml.
• api/authentication.md: refresh tokens are stored in the thelia.cache.security pool, which survives a deployment and a cache clear, with a link to the new page for the backend and the eviction policy.
• getting-started/configuration.md: the config/packages/ listing was missing seven files a 3.0 install actually ships and is now the real one.

Every command on the new page was run on a 3.0 install with demo data, in both modes.

This documents thelia/thelia#3892 and is to be merged with the next core release.

Test plan

• npm ci && npm run build succeeds. The new page renders in both the current and the 3.0 versioned docs, and adds no broken link or broken anchor; the ones the build reports are the pre-existing 2.x loop pages.
• The sidebar of Getting started reads Configuration, Application cache, First Steps.
• Every clear the page describes was counted as files under var/pools/dev, with the catalog pool enabled: cache:clear leaves all 16 entries in place, thelia:cache:clear and the back office button take the 14 of thelia.cache.data_access to 0 and leave the refresh token and the rate limit counter, and the refresh endpoint answers 200 after each.
• docs/ and versioned_docs/version-3.0/ are byte identical for the three pages.