docs: migration guide from Microsoft.Extensions.Http.Resilience

[thomhurst]

Problem

Most ASP.NET Core adopters arrive from Microsoft.Extensions.Http.Resilience (AddStandardResilienceHandler, AddStandardHedgingHandler, AddResilienceHandler), not from raw Polly. Current coverage is one table and two snippets inside docs/docs/polly-migration.md:271-320. Missing:

• HttpStandardResilienceOptions field-by-field mapping (RateLimiter, TotalRequestTimeout, Retry, CircuitBreaker, AttemptTimeout) with default values side by side.
• HttpStandardHedgingResilienceOptions and RoutingStrategyBuilder ordered/weighted groups mapped to HttpEndpointSelectionMode.
• ResilienceHandlerContext and AddResilienceHandler(name, (builder, context) => …) mapped to AddShield((request, serviceProvider) => …).
• RequestMessageSnapshot semantics versus HttpContentReplayPolicy and unsafe-method replay defaults.
• ConfigureHttpClientDefaults usage and the handler-rotation caveat.

Proposed fix

• New page docs/docs/http-resilience-migration.md in the sidebar next to the Polly migration page.
• Move the HTTP section out of polly-migration.md and cross-link.
• All snippets validated by Verify-DocSnippets.ps1; README links the page from the HTTP section.

Tests

• Docs build, snippet compilation, and link check in CI.