Add information about how to set environment variable ExternalLogin__EntraId__ClientSecret

[bankovaprgs]

Parent: https://github.com/telerik/reporting/issues/3647

Suggestions for https://docs.telerik.com/report-server/dotnet-docs/using-microsoft-entra-id

1. Add that if ClientSecret is added in appsettings.json, the Report Server application should be restarted (from IIS if it is used).
2. Instead of using appsettings.json, ExternalLogin__EntraId__ClientSecret environment variable can be set for the respective user. (If IIS is used this is the application pool user, by default ReportServerUser). The environment variable will be applied after the restart of the respective process. If dotnet is used, stop it and start it, if IIS is used, open command prompt as admin user and type "iisreset".

Please note that the__ in the environment variable name are two symbols.

3. Existing tenant can be used if our users find this appropriate. (You can use existing tenant if this better suits your needs.)

Suggestions for https://docs.telerik.com/report-server/dotnet-docs/using-microsoft.entra-id-linux
Explain how to set ExternalLogin__EntraId__ClientSecret

  telerik-report-server:
    environment:
      - ExternalLogin__EntraId__ClientSecret=yoursecretvalue