Port System.Text new docs

[carlossanlop]

@dotnet/area-system-text-encoding @directhex @gewarren PTAL

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-encoding

[gewarren]

But paramrefs need to be updated.

[learn-build-service-prod]

Learn Build status updates of commit 6cd578c:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Text/Ascii.xml	✅Succeeded	View
xml/System.Text/ASCIIEncoding.xml	✅Succeeded	View
xml/System.Text/StringBuilder.xml	✅Succeeded	View
xml/System.Text/UTF8Encoding.xml	✅Succeeded	View

For more details, please refer to the build report.

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