update readme to link to the new polkadot-docs #19948

Job	Run time
check-dependency-rules	9s
check-markdown	19s
check-workspace	20s
check-fail-ci	12s
check-readme	3m 10s
preflight / preflight	51s
preflight / ci-versions	58s
check-toml-format	1m 1s
check-rust-feature-propagation	2m 0s
fmt	1m 7s
check-umbrella	2m 4s
test-rust-features	2m 0s
All quick checks passed	1s
	14m 12s

Provide feedback