docs: fix link and redirect to old docs

docs/.vuepress/public/_redirects

 # redirect v14 urls to root
-/en/start/*          /:splat
-/en/features/*       /:splat
-/en/configurations/* /:splat
-/en/workflow/*       /:splat
+/en/start/*          /guide/:splat
+/en/features/*       /guide/:splat
+/en/configurations/* /guide/:splat
+/en/workflow/*       /guide/:splat
 
 # redirect any other langs to legacy
 /zh-cn/*             https://vue-loader-v14.vuejs.org/zh-cn/:splat

docs/README.md

 # Introduction
 
 :::tip VERSION NOTE
-This is the documentation for Vue Loader v15 and above. If you are upgrading from v14 or an earlier version, check out the [Migration Guide](../migrating.md). If you are using an older version, the old docs are [here](vue-loader-v14.vuejs.org).
+This is the documentation for Vue Loader v15 and above. If you are upgrading from v14 or an earlier version, check out the [Migration Guide](../migrating.md). If you are using an older version, the old docs are [here](https://vue-loader-v14.vuejs.org).
 :::
 
 ## What is Vue Loader?