docs: Rework the Operations section for 4.0

[developerjamiu]

Summary

Section-by-section rework of the Operations concept pages for Serverpod 4.0, following the Data & the database (#696), Web server (#711), and Scheduling (#719) passes. The section gains an entry point, loses the two topics that did not belong to it, and every remaining page was checked against the framework source.

Changes

• New Overview page. The section now opens with what Operations covers and links the pages under it.
• Caching moves to Endpoints & APIs, next to Sessions and Server events, since it is a Session facility rather than a run-in-production concern. The page now documents invalidation, cache groups, the entry limit and eviction order, and the four caches rather than three.
• Experimental features is replaced by Exception monitoring, named for its actual subject. Shutdown tasks move to Running your server, the diagnostic-handler test example moves to Advanced testing examples, and the CLI flag table (whose only row enabled nothing in 4.0) is dropped.
• Logging: separates the four kinds of record, tables the settings with their environment variables and real defaults, and documents runtime log settings and the conditions under which log purging is disabled.
• Health checks: separates the HTTP probes from the metrics collection, which share a name but are unrelated APIs, and corrects the response format and metric cadence.
• Security and TLS: leads with the common case (Serverpod Cloud terminates TLS for you, a proxy usually does otherwise) before the case where the server does it itself.

Notes

• Two code samples in the section did not compile as written and are fixed: the CacheMissHandler example returned a nullable value from a non-nullable method, and the ServerHealthMetric example omitted the required granularity argument.
• /startupz no longer claims to verify migrations or to govern traffic routing; readiness governs routing.
• The global cache falls back to an in-memory, per-instance cache whenever Redis is not configured, in every run mode. This was previously documented as a development-only fallback.
• Cloud logs and the Redis guide now link to their framework counterparts.

[Zfinix]

Caching moving to endpoints-and-apis and experimental-features going away both change URLs, they need redirects too.

[Zfinix]

LGTM 🚀