Remove sphinx-toolbox

[kprosise]

Extension was only being used for the confval domain, which has been builtin to Sphinx since sphinx 7.0, rendering the extension redundant. Additionally, the extension holds back the docs from being updated to a newer Sphinx release.

Checked built output, minor differences in rendering, but overall effect remains the same, and no issues spotted.

This commit applies to FFTK-4730, "Update Sphinx and dependencies"

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