feat: sample diagram and docs for gitgraph (#751)

src/lib/components/preset.svelte

@@ -79,7 +79,19 @@
     section Go home
       Go downstairs: 5: Me
       Sit down: 3: Me
-      `
+      `,
+		'Git Graph': `    gitGraph
+      commit
+      commit
+      branch develop
+      checkout develop
+      commit
+      commit
+      checkout main
+      merge develop
+      commit
+      commit
+`
 	};
 
 	const loadSampleDiagram = (diagramType: string): void => {

src/routes/edit.svelte

@@ -59,6 +59,10 @@
 		},
 		journey: {
 			code: '/#/user-journey'
+		},
+		gitGraph: {
+			code: '/#/gitgraph',
+			config: '/#/gitgraph?id=gitgraph-specific-configuration-options'
 		}
 	};
 	let text = '';