add enterprise (#113)

**before**

![image](/attachments/ec6b488a-12e0-498a-8bba-93b7073991ff)
**after**

![image](/attachments/f886af6c-ad44-4155-b453-c63162c57e56)

Co-authored-by: HEREYUA <1240335630@qq.com>
Reviewed-on: https://gitea.com/gitea/gitea-docusaurus/pulls/113
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: liuyu <liuyu@noreply.gitea.com>
Co-committed-by: liuyu <liuyu@noreply.gitea.com>

docusaurus.config.js

@@ -212,6 +212,13 @@ const config = {
             label: 'Code',
             position: 'left',
           },
+          {
+            position: 'left',
+            label: 'Enterprise',
+            href: 'https://docs.gitea.com/enterprise',
+            className: 'internal-href',
+            target: '_self',
+          },
           {
             type: 'search',
             position: 'right',