Copy and Redirect other Old grails-database-migration Documentation

Example: https://grails.github.io/grails-database-migration/5.0.x/index.html

Source: https://github.com/grails/grails-database-migration/tree/gh-pages
Destination: docs-legacy-database-migration/

To maintain git history

```
git remote add grails-database-migration git@github.com:grails/grails-database-migration.git
git fetch grails-database-migration --no-tags
git merge -s ours --no-commit --allow-unrelated-histories grails-database-migration/gh-pages
git read-tree --prefix=docs-legacy-database-migration/ -u rails-database-migration/gh-pages
git commit -m "Imported grails-database-migration legacy documentation"
```

Steps: https://github.com/apache/grails-website/issues/3

And then redirect snapshot, latest using same method as https://github.com/apache/grails-website/pull/10