Change branch name from master to main in some documents' links (#25126)

As title. And needs to backport to 1.19

docs/content/page/index.fr-fr.md

@@ -70,7 +70,7 @@ Le but de ce projet est de fournir de la manière la plus simple, la plus rapide
     - MSSQL
     - [TiDB](https://github.com/pingcap/tidb) (MySQL protocol)
   - Fichier de configuration
-    - Voir [ici](https://github.com/go-gitea/gitea/blob/master/custom/conf/app.example.ini)
+    - Voir [ici](https://github.com/go-gitea/gitea/blob/main/custom/conf/app.example.ini)
   - Panel d'administration
     - Statistiques
     - Actions

docs/content/page/index.zh-tw.md

@@ -71,7 +71,7 @@ Gitea 是從 [Gogs](http://gogs.io) Fork 出來的，請閱讀部落格文章 [G
     - MSSQL
     - TiDB（MySQL 協議）
   - 設定檔
-    - [app.ini](https://github.com/go-gitea/gitea/blob/master/custom/conf/app.example.ini)
+    - [app.ini](https://github.com/go-gitea/gitea/blob/main/custom/conf/app.example.ini)
   - 管理員面板
     - 系統摘要
     - 維護操作