Consider adding canonicalURL to all older versions

[MartinMalinda]

PR #239 introduces adding canonical URL from the latest version to release and from LTS version to lts. The motivation is to make the release and lts more SEO-important and to make them appear higher in the search results.

An alternative would actually be to have a canonical URL on all versions but latest. This could further help SEO so that latest version is highest. But I am concerned it could make some API from older versions harder or maybe even impossible to google. This strategy is used by the Django docs: https://docs.djangoproject.com/en/1.8/. It seems like Laravel docs do the same: https://laravel.com/docs/4.2/installation

Angular and Vue.js docs do not seem to have any canonical URLs.

https://v1.vuejs.org/api/ https://docs.angularjs.org/guide/concepts

[toddjordan]

May be difficult pre-2.16, since class names/modules have changed. Also would have to avoid broken links when classes/properties/methods drop off/change.