Add language slugs (System.Net)

[gewarren]

Prep for dotnet/docs#45892.

(Hide whitespace changes.)

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl

[learn-build-service-prod]

Learn Build status updates of commit 9aa8886:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http.Headers/AuthenticationHeaderValue.xml	✅Succeeded	View
xml/System.Net.Http/HttpClientHandler.xml	✅Succeeded	View
xml/System.Net.Http/SocketsHttpHandler.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit da2259e:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http.Headers/AuthenticationHeaderValue.xml	✅Succeeded	View
xml/System.Net.Http/HttpClientHandler.xml	✅Succeeded	View
xml/System.Net.Http/SocketsHttpHandler.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel