Janusgraph: Documentation is missing link to API docs

[sjudeng]

The javadoc page should include a link to the API documentation (see corresponding Titan page).

[warrn]

It appears that the javadocs are not even being generated yet, so either a new issue should be created, or this issue should be updated to reflect that

[mbrukman]

The link should point to https://javadoc.io/doc/org.janusgraph/janusgraph-core which will automatically redirect to the latest version, but all versions are accessible there via the dropdown.