Add javadoc documentation in graph service
First pass to add javadoc in the server side of swh-graph.
What needs to be done still:
- Figure out how to link to the swh-graph/docs
- Fix external documentation links (eg: to webgraph framework)
- Add more technical details (eg: Graph, mapping files, etc.)
First step towards T1904.
Migrated from D1741 (view on Phabricator)