Added example using graphs, vertices and edges

[tjoubert]

Solves #183
Added 1 example of how to use graph, vertices and edges in a simple case. See https://github.com/ArangoDB-Community/arangodb-net-standard/blob/feature-3.8/DE-286-graph-get-started/readme.md#example-using-graphs

[DiscoPYF]

Great addition, thanks 👍 Approved.

There is also PostVertexAsync that can be used to create vertex documents for a named graph. Should we mention it? Maybe just a short comment?

The advantage of using PostVertexAsync with a named Graph is that it throws an error with 404 "if no graph with this name could be found. Or if a graph is found but this collection is not part of the graph."

On another hand, maybe it falls too much into specific ArangoDB documentation.