docs: document "sample data" switch and function

(cherry picked from commit b9dfe4f)

Author: ctron

server/src/profile/api.rs

@@ -42,6 +42,7 @@ pub struct Run {
     #[arg(long, env)]
     pub devmode: bool,
 
+    /// Inject example importer configurations during startup
     #[arg(long, env)]
     pub sample_data: bool,
 

server/src/sample_data.rs

@@ -170,6 +170,7 @@ async fn add_quay(
     .await
 }
 
+/// Insert example importers, ignoring existing ones
 pub async fn sample_data(db: trustify_common::db::Database) -> anyhow::Result<()> {
     let importer = ImporterService::new(db);