Copy Old grails-cache Documentation to grails-website repo and then Redirect

The docs should be placed in a new directory named: `docs-legacy-grails-cache`

Copy from: https://github.com/apache/grails-cache/tree/gh-pages to docs-legacy-grails-cache

Then update https://github.com/apache/grails-cache/tree/gh-pages to redirect all https://apache.github.io/grails-cache/ URLS to https://grails.apache.org/docs-legacy-grails-cache/.

`/`, `snapshot` and `latest` should redirect to new locations in combined documentation https://grails.apache.org/docs/snapshot/guide/cache.html

See example:

https://github.com/apache/grails-website/tree/asf-site-production/docs-legacy-views
https://github.com/apache/grails-views/tree/gh-pages