docs: update install instructions to use pnpm and remove `skip-npm` #777

Job	Run time
Lint JS	17s
Tests: Type Check	18s
Tests (addon-location)	1m 17s
Slow tests: defaults with npm	2m 39s
Tests (test-app-location)	51s
Slow tests: typescript with npm	2m 35s
Tests (addon-only)	36s
Slow tests: typescript with yarn	2m 17s
Slow tests: defaults with yarn	1m 28s
Tests (monorepo with npm)	2m 10s
Slow tests: typescript with pnpm	1m 33s
Slow tests: defaults with pnpm	1m 9s
Tests (monorepo with yarn)	1m 17s
Tests (monorepo with pnpm)	50s
Tests (rollup-build)	1m 10s
Tests (declarations-configuration)	39s
	21m 6s

Provide feedback