Add alias definition to /en-us/upgrade-from-gitea page

Since aliases wasn't working locally for me, I also added entries to
static/_redirects file so Netlify will handle the redirects

Fixes go-gitea#7208

Author: lazyguru

docs/content/doc/upgrade/from-gitea.en-us.md

@@ -1,6 +1,8 @@
 ---
 date: "2021-09-02T16:00:00+08:00"
 title: "Upgrade from an old Gitea"
+aliases:
+  - /en-us/upgrade/
 slug: "upgrade-from-gitea"
 weight: 10
 toc: false

docs/static/_redirects

@@ -10,3 +10,7 @@ https://gitea-docs.netlify.com/* https://docs.gitea.io/:splat 302!
 /en-us/ci-cd/ /en-us/integrations/ 302!
 /en-us/third-party-tools/ /en-us/integrations/ 302!
 /en-us/make/ /en-us/hacking-on-gitea/ 302!
+/en-us/upgrade/ /en-us/upgrade-from-gitea/ 302!
+/fr-fr/upgrade/ /fr-fr/upgrade-from-gitea/ 302!
+/zh-cn/upgrade/ /zh-cn/upgrade-from-gitea/ 302!
+/zh-tw/upgrade/ /zh-tw/upgrade-from-gitea/ 302!