Update screenshots for cloud once theme is merged

[arnavs]

[WenxinM]

One issue in the Cloud setup is the launching server. We used to have three servers: the pims syzygy, ubc syzygy, and google colab, and the first instruction in the current Cloud setup (including screenshots) is based on the pims syzygy. The servers available now are google colab, and the binderhub, the interface of which looks slightly different.

If we don't want to use the syzygy servers anymore, then the instructions and the screenshots need to be updated

[WenxinM]

For lecture Local installation, there are at least the following that we should update:

• [x] The latest Python version is 3.9 and the current version is based on the 2019 launch.
• [ ] Screenshot for Downloading the QuantEcon Data Science Lectures (the clone button)
• [x] The path for git clone

[arnavs]

Removed syzygy mention, screenshots waiting on new theme

[WenxinM]

See #95 for updates related to the servers