CI/DOC: allow overriding Ergast backend for doc build #1001

Job	Run time
Test import order (isort)	2m 58s
Linting (Ruff)	59s
Tests on 3.9 (Minimum Versions)	46m 13s
Test readme renders on PyPi	16s
Tests on 3.9	14m 17s
Tests on 3.10	15m 22s
Tests on 3.11	13m 58s
Tests on 3.12	18m 16s
Test Docs / Build Documentation	47m 40s
	2h 39m 59s

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