Allow admin themes to customize field hints

[Klabos]

Summary

• introduce a Hint_Admin shape in TheAdmin while preserving the existing hint markup by default
• render configurable content field hints through the shape so custom admin themes can override their presentation
• document the new admin theme extension point and an accessible tooltip example

Testing

• dotnet build OrchardCore.slnx -c Release
• dotnet build src/OrchardCore.Themes/TheAdmin/TheAdmin.csproj -c Release --no-restore
• manually verified the default hint rendering and custom shape override

Fixes #18826

[Klabos]

@dotnet-policy-service agree