convert "Get started ..." docs over to "Step 1 ..."-style

A suggestion from @bmorelli25 (from https://github.com/elastic/apm-agent-nodejs/pull/2959#discussion_r1005822973) is that we convert our "Get started with ..." documents over to a "Step 1: ...", "Step 2: ...", ... style for clarity. The coming Next.js doc will be using that style. The [Lambda doc](https://github.com/elastic/apm-agent-nodejs/pull/2959#discussion_r1005822973) already uses that style.