doc: humongous caller / callee graphs

The guys at Doxygen provided some food for the brain:

When looking at the warning we also see warnings about not generated graphs like: Caller graph for 'ns3::Time::GetSeconds' not generated, too many nodes (223), threshold is 70. Consider increasing DOT_GRAPH_MAX_NODES.

It makes sense to have the number of nodes in a graph limited as otherwise they get (especially with fixed formatted output) unreadable. When having HTML output it is worth to set DOT_IMAGE_FORMAT to svg and also to set INTERACTIVE_SVG=YES. Furthermore when one has really big graphs (like for ns3::Simulator::Now) one should have a look at the commands like \hidecallgraph / \hidecallergraph to selectively disable these graphs.

This is actually a very good comment. Humongous claller / callee graphs are not useful to anybody, they clutter the documentation, they generate warnings, and they slow down the doxygen generation.

A good MR should:

1. Review the warnings,
2. Propose which classes should have their graph hidden,
3. Hide these graphs and add a sentence in the documentation stating that the graph is hidden on purpose.
4. Change the image formato to SVG / interactive.