Expand documentation on `[Obsolete]` static APIs and sync wrappers,
emphasizing their maintained status and appropriate use cases.
- Added detailed examples for `BuildStaticClient` and `Create`
in VB.NET and C#, including configuration options.
- Updated `TaskSyncExtensions.Sync` section with warnings about
potential deadlocks and recommendations for `async/await`.
- Introduced "6.3 Mid-Term Recommendation" to guide migration
to DI and async patterns.
- Highlighted scenarios where static APIs and sync wrappers
remain appropriate, such as legacy .NET Framework projects
or quick-start use cases.
- Clarified that `[Obsolete]` is a reminder, not a breaking change.
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