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)