[Http Client Latency] Add new page describing the HttpClientLatencyExtension methods

[rainsxng]

Summary

This PR adds a new page describing the HttpClientLatency extension methods. This API was long presented in the dotnet/extensions, but it was never documented except for the readme in the project itself. The latest changes were merged in dotnet/extensions#6783, so this PR reflects those changes as well

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Internal previews

📄 File	🔗 Preview link
docs/core/extensions/httpclient-latency-extensions.md	docs/core/extensions/httpclient-latency-extensions
docs/fundamentals/toc.yml	docs/fundamentals/toc

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for the HttpClientLatency extension methods, which provide detailed timing information for HTTP client requests. The documentation covers configuration, usage patterns, and integration with other related services.

• Adds a new documentation page explaining HTTP client latency telemetry features and configuration
• Updates the table of contents to include the new documentation page
• Provides code examples for registration, configuration, and usage of the telemetry features

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/fundamentals/toc.yml	Adds navigation entry for the new HTTP client performance monitoring documentation
docs/core/extensions/httpclient-latency-extensions.md	Creates comprehensive documentation covering HTTP client latency telemetry configuration, usage, and platform considerations