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)

Start by locating how documentation URLs and versioned paths are generated in this repository, then inspect the routing or redirect configuration for unversioned links. Compare the current and v4 examples from the issue; done means versioned documentation URLs are used consistently and unversioned URLs redirect to the current version without breaking existing links.

索引モデルが issue の本文から書いたものです。