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docs: polish onboarding guides and surface them in README #18256

Job	Run time
changes	26s
kernel-static-checks	0s
chat-bundle	0s
typescript-core (${{ matrix.shard }}/3)	0s
dashboard-browser	0s
node-forward-compatibility	-1s
presentation	-1s
kernel-contracts	-1s
node-minimum-compatibility	-1s
chat-bundle-browser	-1s
test-shard	-1s
merge-gate	14s
stage2c (${{ matrix.suite }} ${{ matrix.shard }})	-1s
dashboard-acceptance	-1s
stage2c-correctness-e2e	-1s
typescript-coverage	-1s
windows-powershell	-1s
checks	-1s
pytest	-1s
sonar	-1s
	26s