[FEATURE]

**ECS integration/library project(s) (e.g. Elastic.CommonSchema.Serilog)**:

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
I want a migration guide from Serilog.Sinks.Elasticsearch where we use a in house ElasticSerch server. We configyre the UR and an IndexFormat

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
I want our new implementation write loggs to the server as the old one did

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.
The old implementation:
```
public class FastProLogger : IFastProLogger
{
private readonly ILogger _seriLogger;

public FastProLogger(IConfiguration configuration)
{
string elasticSearchIndexFormat = configuration["ElasticSearch:IndexFormat"] ?? throw new ApplicationException($"Konfiguration för ElasticSearch:Uri saknas");
string unused = configuration["Logging:LoggDumpFilePath"] ?? throw new ApplicationException($"Konfiguration för Logging:LoggDumpFilePath saknas");
string elasticSearchUri = configuration["ElasticSearch:Uri"] ?? throw new ApplicationException($"Konfiguration för ElasticSearch:Uri saknas");
IConfigurationSection section = configuration.GetSection("ElasticSearch:IndexFormat");
string? sectionValue = section.Value;

ElasticsearchSinkOptions options = new(new Uri(elasticSearchUri))
{
AutoRegisterTemplate = false, // we do not appear to have enough access rights to register templates
AutoRegisterTemplateVersion = AutoRegisterTemplateVersion.ESv6,
IndexFormat = elasticSearchIndexFormat,
EmitEventFailure = EmitEventFailureHandling.ThrowException,
RegisterTemplateFailure = RegisterTemplateRecovery.FailSink,
CustomFormatter = new ElasticsearchJsonFormatter()
};

_seriLogger = new LoggerConfiguration()
.MinimumLevel.Debug()
.MinimumLevel.Override("Microsoft", LogEventLevel.Information)
.Enrich.FromLogContext()
.WriteTo.Elasticsearch(options)
.CreateLogger();

}

public void Debug(string messageTemplate, params object[] propertyValues)
{
_seriLogger.Debug(messageTemplate, propertyValues);
}
...
}

```

No target documentation file or test is named. Start by reviewing the ECS integration/library documentation and the existing Serilog.Sinks.Elasticsearch configuration in the issue, then identify where a migration guide belongs. Done means documenting how the new implementation maps the old URI, IndexFormat, template, formatter, and failure-handling settings.

Written by the indexing model from the issue text.