documentation: clarify which steps are optional vs mandatory

[webern]

What I'd like:

As recommended by @tjkirch in #899: we should re-organize the setup steps to clarify which steps are always required vs. those that are optional. Specifically

it'd be even better to move the CNI step into the "cluster creation" block to clarify that it's mandatory, and to rename "Remaining cluster configuration" to "Optional cluster configuration" to clarify that this new bit and conntrack aren't required. As we get more steps in our quickstart, I think it's important to emphasize the ones that are actually needed by all users.

Any alternatives you've considered:

N/A