Clarify differences between subscription levels and feature tiers

In https://github.com/elastic/docs-content/issues/2768, I cleaned up occurrences but didn't really think we'd need actual written guidance on this (this is how our products work...), nor thought about clarifying this page that is tagged as serverless but more generally ignoring the details about serverless.

This needs to be clarified because feature availability doesn't depend on subscription levels for serverless users. Here's an extract from a conversation I had on Slack last month:

• Serverless feature tiers are not subscription levels
• Serverless feature tiers are project by project, they determine the features available to you in that specific project only
• Subscription levels are for the entire account, they determine the support level for the entire account (including serverless), and the features available to you in all your hosted deployments
• That means that even serverless users can have a cloud subscription on top of their serverless project feature tier :dizzy_face:. So when they read the requirements, we need to make sure they don't think that they need both when they don't

At that time and for that specific thread, I ended up recommending something like this, though we can adapt to be usable more broadly and/or to also take account occurrences when we do need to list the exact subscription/feature tier needed, like we had more recently with Agent Builder: This functionality is available in {{kib}} deployments and serverless projects with an appropriate [subscription or feature tier](https://www.elastic.co/pricing).

I'll label this as admin for now since this is more of an admin topic to begin with, but open to collab between teams as this might impact quite a few docs