Add reference docs for the load generator

We should add reference docs that describe relevant information about the load generator from a user perspective (this needs to be a new page in the docs). Two aspects are:

* [x] Create charts explaining the dynamics of target-throughput/interval / num of clients / bulk_size / iterations / schedule - owned by @dliappis
* [ ] move the [scheduler docs from the track reference](https://esrally.readthedocs.io/en/stable/track.html#choosing-a-schedule) to the new page and add charts from ^^ item - owned by @hub-cap
* [ ] Provide a high-level introduction and specifically document how clients are assigned to OS processes and load generator hosts - owned by @hub-cap
* [ ] Measure and document how throughput / number of active clients as a function of the number of clients specified - owned by @ebadyano

Start with the scheduler documentation in the linked track reference and identify the new documentation page where it should move. Add the requested load-generator charts, high-level client and host/process explanation, and measured throughput and active-client results; done means the new page covers all listed aspects and the old scheduler reference points to it.

Written by the indexing model from the issue text.