Update global time series plotting docs

[forsyth2]

Improve documentation on adding a new plot to Global Time Series.

[forsyth2]

@evasinha Add any comments here on what would be helpful to include in the plotting docs. Once we successfully merge a new plot into zppy, we can merge this doc update. Thanks!

[evasinha]

@forsyth2 - Maybe adding something similar to this:

After making edits to your zppy fork, use the following pre-commit commands described earlier before generating plots with your edits:

pre-commit install
pip install .