Offline Documentation Download for AI training

### Feature description

AI assistants increasingly rely on local, chunkable documentation. Please make the latest (and each released) Grails documentation downloadable as archives so it can be embedded in tooling/LLM context windows. This should include (1) the Reference Guide (narrative docs) and (2) the API docs (Javadoc) for Grails core and first-party plugins.

Start by reviewing the existing publication process for the Reference Guide and Javadoc for Grails core and first-party plugins. Define archive contents and locations for the latest release and each released version, then verify that both documentation sets can be downloaded and consumed as archives.

Written by the indexing model from the issue text.