Enhance ordering of tutorials in sphinx gallery

[oruebel]

Motivation

Sphinx gallery currently sorts tutorials within a section based on file size. This order is not very intuitive.

Changes in this PR

• Updated sorting of tutorials within a section to be alphabetical based on the section headings
• Updated tutorial headings as necessary to use consistent capitalization and be consistent with the icons
• Minor changes to headings to improve ordering
• Updated ophys and icephys tutorial to use relative width for figures to ensure consistent rendering and avoid too large images in LaTeX

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1399 (1849790) into dev (e484bfd) will decrease coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 1849790 differs from pull request most recent head e56c3d2. Consider uploading reports for the commit e56c3d2 to get more accurate results

@@            Coverage Diff             @@
##              dev    #1399      +/-   ##
==========================================
- Coverage   77.31%   77.26%   -0.05%     
==========================================
  Files          37       37              
  Lines        2715     2714       -1     
  Branches      450      450              
==========================================
- Hits         2099     2097       -2     
  Misses        535      535              
- Partials       81       82       +1     

Impacted Files	Coverage Δ
src/pynwb/icephys.py	89.52% <0.00%> (-0.35%)	⬇️

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[oruebel]

@rly this is ready for review

[rly]

@oruebel Is this ready to merge? If so, please Squash and Merge. Thanks