[mdatagen] Add description field to samplefactoryreceiver

Move the description text inside the autogenerated section by adding
the description field to metadata.yaml. This ensures all sample
components follow the same pattern where description is generated
from metadata rather than manually written outside the markers.

Related to #14114

Author: thc1006

cmd/mdatagen/internal/samplefactoryreceiver/README.md

@@ -1,7 +1,8 @@
-This receiver is used for testing purposes to check the output of mdatagen.
-
 <!-- status autogenerated section -->
 # Sample Factory Receiver
+This receiver is used for testing purposes to check the output of mdatagen.
+
+
 | Status        |           |
 | ------------- |-----------|
 | Stability     | [deprecated]: profiles   |

cmd/mdatagen/internal/samplefactoryreceiver/metadata.yaml

@@ -2,6 +2,7 @@
 
 type: sample
 display_name: Sample Factory Receiver
+description: This receiver is used for testing purposes to check the output of mdatagen.
 scope_name: go.opentelemetry.io/collector/internal/receiver/samplefactoryreceiver
 github_project: open-telemetry/opentelemetry-collector