Domain decomposition and simulation C++ API docs (#471)

Add two new documentation pages for the C++ API

* Add domain decomposition page that covers `domain_decomposition`, `node_info` and `partition_load_balance`.
* Add simulation page that describes `arb::simulation` API interface.
* Fix some small typos elsewhere in the docs.
* Use `std::move` when adding spike callbacks to `arb::simulation` (useful if callbacks are stateful).