Docs updates

[amithkoujalgi]

• Updated docs GH workflow to build and publish javadoc alongside of docs directory. This would make the docs available at https://langchain4j.github.io/langchain4j and javadoc at https://langchain4j.github.io/langchain4j/apidocs/index.html
• Updated javadoc build command in docs GH workflow to do a full project build (that includes all modules and no exclusions).
• Updated javadoc build in docs GH workflow to run with Java 21 version.

This is in reference to #596

[LizeRaes]

Great, thanks for solving this!