V5 tracker: Final release-facing docs and sample refresh after v5 implementation stabilizes

[cwinland]

Docs reference: README.md, docs/**/*.md, samples, and release-facing content

This tracker is the final v5 release-facing documentation pass. It should happen after the v5 feature, generator, provider-expansion, and cleanup milestones are complete enough that the shipped surface is real and stable.

Scope:

• update the root README, getting-started guidance, provider-selection and provider-capabilities docs, feature-parity claims, cookbook entries, samples, and what's-new or release-note content for the shipped v5 surface
• remove or rewrite any v4 guidance that becomes inaccurate once the v5 package shape, generator surface, provider lineup, and cleanup work are complete
• document only the v5 behavior that is actually implemented and ready to ship
• coordinate the final public documentation pass across the generator work, provider expansion, compatibility cleanup, and MockOptional retirement instead of documenting those slices piecemeal too early

Non-goals:

• using release-facing docs to design or pre-announce v5 features before implementation is ready
• replacing the current v4 documentation cleanup issues, which should stay focused on the existing and near-term surface

Acceptance criteria:

• v5 release-facing docs reflect the shipped package shape, provider defaults, generator capabilities, provider expansion, and cleanup changes accurately
• outdated v4-only guidance that would mislead v5 readers is removed or rewritten
• samples and what's-new style content align with the actual released v5 behavior rather than roadmap intent
• the documentation pass happens after the relevant v5 implementation work is complete enough to support stable release-facing guidance