Add examples for multi proc codegen

[proppy]

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.