Document how PyACP can be used to modify a WB project in-place

### Description of the modifications

Should contain the following info:
- how to locate / open the `.acph5` files
- a note not to use ACP GUI and PyACP _simultaneously_ on the same file
- instructions on how to trigger the WB cell to be out-of-date

The issue does not identify a documentation file or entry point, so first locate the existing PyACP documentation for working with WB projects. Document how to locate and open .acph5 files, warn against simultaneous ACP GUI and PyACP access to the same file, and explain how to trigger a WB cell to become out of date.

Written by the indexing model from the issue text.