Update Service Fabric version guidance for the lowered dependency floor

[tillig]

Follow-up to #188. Merge this with the Autofac.ServiceFabric 5.0.0 release, not before — it describes behavior that ships in autofac/Autofac.ServiceFabric#71.

Why

#188 told consumers to keep their Service Fabric packages "on the same version as the one Autofac.ServiceFabric depends on." That was accurate at the time, but autofac/Autofac.ServiceFabric#71 lowers the integration's dependency floor to the oldest supported SDK precisely so consumers aren't pinned to our version. The old advice becomes wrong.

What changed

• Consumers choose their own Service Fabric version; the integration floors at the oldest supported SDK and NuGet lets the higher version win.
• The thing they actually need to do is keep their own Service Fabric references consistent with each other — that's what produces NU1608. The example warning is kept, since it's still exactly what a mismatch looks like.
• Added the consequence of not referencing the Service Fabric packages at all: you silently get the oldest supported versions. Also noted the project templates add them for you.
• Added a note that building against an older SDK than you run against is expected, because the Service Fabric assemblies keep a stable assembly version across package releases, with a link to Microsoft's SDK/runtime pairing table.

Verification

doc8 clean. Full sphinx -b html build succeeds and the new content renders. The only warnings are the two pre-existing unknown document: '../troubleshooting/diagnostics' references in faq/container-analysis.rst and lifetime/disposal.rst.