auto generate and inject gallery carousel

[bjlittle]

📰 Custom Issue

The gallery carousel introduced in #828 was manually created for the examples already existing in the sphinx-gallery.

It is possible to automate this such that the RST to render the carousel is created and injected to the root index.rst whenever we make the documentation.

This will give us a more robust carousel that is in sync with the gallery.

TODO:

• [x] Remove tags.plot_carousel in conf.py for use with only directive in src/index.rst
• [x] Remove suppress_warnings = ["image.not_readable"] in conf.py

[bjlittle]

@all-contributors please add @bjlittle for data, example, question and test

[allcontributors[bot]]

@bjlittle

I've put up a pull request to add @bjlittle! :tada: