Upgrade sphinx version by removing sphinx_panels dependency

[JBGreisman]

The documentation used a deprecated sphinx extension called sphinx_panels. This update switches to the updated version called sphinx_design. This should make it so that a later version of sphinx can be used for building documentation, which resolves the jinja2 version issue from #202 and as a side effect also resolves the issue identified in #206.

This worked locally without the need for setting an explicit minimal sphinx version in setup.py, but it might behave differently in the CI.

[JBGreisman]

Based on the documentation test, the changes here did indeed boost the sphinx version from 3.5.3 --> 5.3.0

[codecov-commenter]

Codecov Report

Merging #208 (e1a22aa) into main (9294621) will not change coverage.
The diff coverage is n/a.

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@@           Coverage Diff           @@
##             main     #208   +/-   ##
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  Coverage   98.39%   98.39%           
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  Files          45       45           
  Lines        1803     1803           
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  Hits         1774     1774           
  Misses         29       29           

Flag	Coverage Δ
unittests	98.39% <ø> (ø)

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