Package and overview html javadocs

[liach]

With #1268, we now have javadocs generating for yarn. However, the javadoc suffers from a few setbacks:

• [ ] Inconsistent titles (See the javadoc jar's index html title vs local build's title); needs a better name than yarn api (also need version string etc)
• [x] #2035 Missing package infos (may be a good place for describing the functionalities of each package)
• [ ] Missing an overview (can use something like pandoc to convert readme markdown to html for javadoc to use, or just make an html page for general readme purposes)
• [ ] Custom help page (can be similar to overview)