docs: Pin sphinx-rtd-theme version fixing build error

[matejmatuska]

Previously the 0.4.3 version was correctly used. There are still some warnings during build, but those were also present previously (verified by checking old RTD build).

Follows up on #876 - fixes required to make build on RTD working again.

[github-actions]

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[pirat89]

lgtm again :D