docs: auto-generate gallery carousel

[bjlittle]

🚀 Pull Request

Description

This pull-request auto-generates the documentation landing page gallery carousel, but only if the sphinx-gallery has built the gallery of examples.

It achieves this by registering a callback with sphinx to generate sphinx-design RST in the docs/src/gallery-carousel.txt file, which is simply included in the docs/src/index.rst using the include directive.

This now achieves a reasonable level of examples automation. By adding an example to the geovista.examples, it will automatically be:

• Built and included in the documentation gallery
• Executable from the CLI entry point e.g., geovista examples --run X
• Image tested as part of our GHA CI and via pytest on the CLI
• Included in the documentation gallery carousel

Sweet.

Closes #846

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[codecov[bot]]

Codecov Report

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

Project coverage is 91.19%. Comparing base (741e767) to head (9a023b4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #867 +/- ## ======================================= Coverage 91.19% 91.19% ======================================= Files 58 58 Lines 2943 2943 ======================================= Hits 2684 2684 Misses 259 259 ```

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