Add documentation example for xconfmap (#5675) (#12832)

#### Description

This PR introduces a simple testable examples to the package
[confmap](/confmap/xconfmap). it's basically a copy of the documentation
example on
https://opentelemetry.io/docs/collector/building/receiver/#designing-and-validating-receiver-settings

#### Issue:
#5675 (comment)

#### Testing

the testable example was executed successfully


![image](https://github.com/user-attachments/assets/1b992f1a-fd69-452d-be46-288672710334)


#### Documentation

this testable example follows
https://go.dev/blog/examples#larger-examples format and should be
automatically added to `xconfmap` package documentation

Co-authored-by: Pablo Baeyens <[email protected]>
Co-authored-by: Pablo Baeyens <[email protected]>

Author: heitorganzeli

confmap/xconfmap/example_test.go

@@ -0,0 +1,62 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package xconfmap
+
+import (
+	"errors"
+	"fmt"
+	"time"
+)
+
+// Config represents the receiver config settings within the collector's config.yaml
+type Config struct {
+	Interval       time.Duration `mapstructure:"interval"`
+	NumberOfTraces int           `mapstructure:"number_of_traces"`
+}
+
+// Validate checks if the receiver configuration is valid
+// this function is automatically called by the collector when it loads the configurations
+// for a component
+func (cfg *Config) Validate() error {
+	if cfg.Interval.Minutes() < 1 {
+		return errors.New("when defined, the interval has to be set to at least 1 minute (1m)")
+	}
+
+	if cfg.NumberOfTraces < 1 {
+		return errors.New("number_of_traces must be greater or equal to 1")
+	}
+	return nil
+}
+
+// Example usage validated configuration
+func Example() {
+	// invalid number of traces
+	myCfg := Config{
+		Interval:       time.Minute,
+		NumberOfTraces: 0,
+	}
+	err := myCfg.Validate()
+	fmt.Println(err)
+
+	// invalid interval
+	myCfg = Config{
+		Interval:       time.Second,
+		NumberOfTraces: 1,
+	}
+	err = myCfg.Validate()
+	fmt.Println(err)
+
+	// valid config
+	myCfg = Config{
+		Interval:       time.Minute,
+		NumberOfTraces: 1,
+	}
+	err = myCfg.Validate()
+	fmt.Println(err)
+
+	// Output:
+	// number_of_traces must be greater or equal to 1
+	// when defined, the interval has to be set to at least 1 minute (1m)
+	// <nil>
+}