Add example showing plotting brain regions in a single hemisphere

[adamltyson]

It's straightforward, but only if you know how to do it!

Based on https://forum.image.sc/t/showing-one-hemisphere-only/101736

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.71%. Comparing base (7952179) to head (2262f9c). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #383 +/- ## ========================================== - Coverage 87.85% 86.71% -1.15% ========================================== Files 27 27 Lines 1219 1219 ========================================== - Hits 1071 1057 -14 - Misses 148 162 +14 ```

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