[JSDK-48] Javadoc and doc improvements

[tl-andrea-dilisio]

• Ships our library javadoc and deploys it to gh-pages branch, thus making it available on https://truelayer.github.io/truelayer-java (it won't be publicly available until we make the repo public ) ;
• Improves DRY on CI workflows;
• Simplifies and completes our README with a few badges.
• Removes the shadowJar plugin: we don't need a fat jar