Add examples for multi proc codegen

Now that most (if not all?) parts of #950 have landed, we should properly document how to use it w/ relevant examples.

https://google.github.io/xls/codegen_options/#codegen-mapping already shows the `--multi_proc` option but as discussed with @grebe there is a few gotcha.

- fifo depth need to be set for all channels instanciations (see #1343)
- a `xls_fifo_wrapper` need to be provided by the user as a sibling system verilog module.