build developer documentation for swh-graph

We need to generate the developer doc for both the Java and Python parts of swh-graph.

The Python part should be business as usual. The Java part should be built with Maven (mvn site). To that end we should:

write such doc!… as it doesn't seem to exist yet :-)
figure out how to integrate the maven-built doc with our sphinx-based documentation pipeline. sphinx-maven-plugin seems the way to go, but I haven't explored it much yet

Migrated from T1904 (view on Phabricator)

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