chore: do not generate docs preview if not on PR #6285

Job	Run time
setup / start-builder	9s
setup / setup	10s
build	2m 32s
bb-native-tests	11s
noir-projects	1m 33s
yarn-project-formatting	1m 24s
bb-bench-binaries	1m 58s
docs-preview	4m 4s
yarn-project-test	53s
setup-bench / start-builder	1m 7s
e2e (e2e-browser)	1m 25s
e2e (e2e-docs-examples)	57s
e2e (e2e-sandbox-example)	1m 29s
e2e (e2e-tests)	21s
e2e (flakey-e2e-tests)	4m 31s
e2e (guides-dapp-testing)	2m 46s
e2e (guides-writing-an-account-contract)	1m 4s
e2e (integration-l1-publisher)	1m 49s
e2e (pxe)	40s
e2e (uniswap-trade-on-l1-from-l2)	5m 47s
setup-bench / setup	29s
bb-bench	15s
merge-check	1s
notify	0s
	35m 35s

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