Copy Old grails-grails-gradle-plugin Documentation to grails-website repo and then Redirect

This is API docs only.

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

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

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

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

See example:

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