docs: improve enterprise portal docs with visual imagery

Our docs are very well rounded in many places, catering to all kinds of learners (book learners, visual learners with pictures, hands-on learners with examples and tutorials).

There are a few gaps here or there, but it's by-and-large a poster example of writing docs for all learning types.

One gap that stood out to me was that some of the Enterprise Portal docs don't have as many screenshots for visual learners.

Ex: https://docs.replicated.com/embedded-cluster/v3/installing-embedded-air-gap

#5 is excellent, but it might be an improvement for visual learners to add some more screenshots, for instance #4 mentions "At the top", and a screenshot would help drive it home.

There's a hidden feature I think with the visual imagery, too. When you're working on something and scanning tons of docs, when you see a screenshot matching what it is your currently working through, it immediately registers that you're on the right page. Whereas if the details are hidden in text, sometimes it may be missed initially by a visual type.

Start with the Enterprise Portal page at docs.replicated.com/embedded-cluster/v3/installing-embedded-air-gap and compare the numbered installation steps, especially #4 and #5. Identify places where the text refers to screen locations such as "At the top" and add screenshots that make those steps easier to follow; done means the relevant Enterprise Portal docs provide useful visual guidance for visual learners.