Add Embedding API docs

* Add apis/ directory for embedding API javadocs (and potentially others in future)
* Move existing "APIs" page to "Web APIs"
* Add new "Embedding API" page, which links to embedding API javadocs
* Reconstruct "APIs" page so it summarises the kinds of APIs available for Crosswalk

Fixes https://crosswalk-project.org/jira/browse/XWALK-1646