chore: add meta theme-color to the docs (#1358)

Co-authored-by: Kia Ishii <kia.king.08@gmail.com>

docs/.vitepress/config.ts

@@ -9,6 +9,8 @@ export default defineConfig({
   lastUpdated: true,
   cleanUrls: 'without-subfolders',
 
+  head: [['meta', { name: 'theme-color', content: '#3c8772' }]],
+
   markdown: {
     headers: {
       level: [0, 0]