Documentation urls should always include version number

Is your feature request related to a problem? Please describe.
Old blog or forum posts, stackoverflow that links to the ionic docs often point to the wrong version of the documentation.
Examples :

• https://ionicframework.com/blog/introducing-ionic-4-ionic-for-everyone/ the link to the Migration guide is https://ionicframework.com/docs/building/migration/ but should be https://ionicframework.com/docs/v4/building/migration/
• https://stackoverflow.com/a/55202668/2765126 the link to the css-custom-properties links to the current doc that includes more properties than in v4 so it's confusing

Describe the solution you'd like

• Always include the version number in the documentation urls (even the current version)
• If no version is added redirect to current version (to prevent dead links)

首先定位此仓库中如何生成文档 URL 和带版本的路径，然后检查针对无版本链接的路由或重定向配置。比较 issue 中当前版本和 v4 的示例；当文档 URL 一致地使用版本路径，并且无版本 URL 在不破坏现有链接的情况下重定向到当前版本时，即表示完成。