DOCS: Add gallery carousel to docs homepage

[tkknight]

DRAFT - DO NOT REVIEW

🚀 Pull Request

Description

Add a carousel for the gallery on the docs homepage. Note:

• Needed some code in the conf.py to create the list of cards to use.
• Needed to pin the pydata-sphinx-theme to avoid weird image overlays. See DOCS: gallery carousel images blanking #6885
• I have removed flake8 too as it flagged issues but were not really issues. We have ruff now so I see no issue in removing flake8.
• A user may need to scroll down to see the gallery carousel, I think we can reduce the size of the big 6 cards to achieve this - not in this PR though.

Inspiration for this taken from the GeoVista Docs, credit to @bjlittle for this 🤜 🤛

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[scitools-ci]

• Please see: SciTools/.github#246

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.43%. Comparing base (3b5fd8c) to head (32ce4bb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6884   +/-   ##
=======================================
  Coverage   90.43%   90.43%           
=======================================
  Files          91       91           
  Lines       24839    24839           
  Branches     4656     4656           
=======================================
  Hits        22463    22463           
  Misses       1608     1608           
  Partials      768      768           

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