Replace custom roles with styled extlinks

[hugovk]

We can replace the custom roles used for cpy-file and gh-label with extlinks, which removes a fair amount of code. One thing, though, the custom roles had code formatting, and extlinks have no special formatting by default.

However, with the newly-released Sphinx 7.4.0, a unique class is added to each extlink, so we can now add our own styling.

• https://www.sphinx-doc.org/en/master/changes.html#release-7-4-0-released-jul-15-2024
• sphinx.ext.extlinks: Add extlink-{name} CSS class to links sphinx-doc/sphinx#12319
• sphinx.ext.extlinks: option for literal formatting? sphinx-doc/sphinx#11745

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