Generate separate Javadocs .jar release artifact

[sreuland]

What problem does your feature solve?

No pre-built javadocs are provided per release

What would you like to see?

a stellar-sdk-javadocs.jar is built by gradle and published as another artifact to jitpack maven repo classifier=javadoc and file on GH releases. having pre-built docs may enable community to readily pair the docs with code while in IDE's for development.

What alternatives are there?

users can run ./gradlew javadoc to generate javadoc content locally.

[overcat]

Implemented in #519.