Update Azure SDK proxy server configuration doc

[scottaddie]

Update the proxy doc to be less hand wavy, using both https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/core/Azure.Core/samples/Configuration.md#configuring-a-proxy and https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/core/Azure.Core/src/Pipeline/Internal/HttpEnvironmentProxy.cs as resources.

Summary of changes

• Document proxy configuration via code.
• Document the ALL_PROXY, NO_PROXY, and GATEWAY_INTERFACE env vars.
• Simplify the URL and add a redirect. The "azure-sdk-" prefix on the file name provided no value.
• Update ToC entry text, along with the doc title and description metadata.

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Internal previews

📄 File	🔗 Preview link
docs/azure/sdk/configure-proxy.md	docs/azure/sdk/configure-proxy
docs/azure/TOC.yml	docs/azure/TOC

[annelo-msft]

This looks consistent to me with the docs we have in https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/core/Azure.Core/samples/Configuration.md#configuring-a-proxy, and modulo the one "consider" comment I added, I think this is a good update to the MS Learn docs. Thanks, @scottaddie!