Docs: Change main spatial figures to 90-zone map

[patrickbrown4]

Summary

This PR updates some of the zone-map figures in the docs for 90-zone resolution, matching the current defaults after #134.

Technical details

Implementation notes

• Added some tweaks to maps.py to reduce label overlap in the 90-zone map

Known incompatibilities

• Some of the maps in the docs still have the old zone outlines, but they're pretty inconspicuous and (to me at least) don't seem to warrant a global update. My plan is just to switch them to state outlines the next time we need to update the plots (e.g. the next time we update the supply curves, transmission costs, etc.).

Validation, testing, and comparison report(s)

• Only figures and .md files were modified (aside from the single maps.py used to generate the new maps), so I didn't run any tests.

Checklist for author

Details to double-check

• Documentation updated if necessary

General information to guide review

• Zero impact on results of default case
• No large data file(s) added/modified
• No substantive impact on runtime for full-US reference case
• No substantive impact on folder size for full-US reference case
• No change to process flow (runreeds.py, reeds/core/solve/solve.py)
• No change to code organization
• No change to package requirements (environment.yml or Project.toml)