Documentation: Compress Image File Size

[jinningwang]

The screenshots added to the documentation turned out to be too large:

1. Built-in cases map at https://github.com/CURENT/agvis/tree/master/docs/source/getting_started/cases
2. Screenshots to demonstrate usage at https://github.com/CURENT/agvis/tree/master/docs/source/usage/diagrams

This enlarges the package size and thus prevents us from publishing it in PyPi. It also slows down the documentation loading speed.

We might need to replace these figures with compressed ones to address this issue.