Draft PL and PA documents for Verilator modeling.

Created by: jeremybennett

The PL document that existed was very out-of-date, so has largely
been written and simplified.

Project-Descriptions-and-Plans/

* MCU-Verilator-Model/verilator-modeling-pa.md: Created.
* MCU-Verilator-Model/verilator-modeling-pl.md: Updated.