Add example code for PMax campaigns to add search theme signal

[yangda]

Context: currently the add_performance_max_campaign.py example only contains code samples to add the audience signal as an asset group signal. We also support adding search themes as an asset group signal, which is documented here. With that we want to modify our code example to include it.

Changes:

1. Added code to demonstrate adding a search theme asset group signal.
2. Updated the document links in comments.
3. Updated the positions of utility comments ([START/END add_performance_max_campaign_*]) to prepare for devsite updates.