Update docs for changes in add fleet server steps

[dedemorton]

Closes issue #1843

I've also applied some minor consistency edits to the content that I touched in this PR.

remaining TODOs:

• Fix my test environ and test again
• Update stale screen captures:
  • https://www.elastic.co/guide/en/fleet/8.3/fleet-server-scalability.html
  • https://www.elastic.co/guide/en/fleet/8.3/fleet-server-monitoring.html

[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[apmmachine]

A documentation preview will be available soon:

• 📚 HTML diff
• 📘 Fleet and Elastic Agent Guide
• 📙 Observability Guide

[colleenmcginnis]

👋 @hop-dev can you please take a look at this PR ahead of Tuesday's release?

[hop-dev]

The content looks good, I am not sure if you're happy to merge with the CPU and memory metrics missing, I doubt we will be able to get that resolved for tomorrow.

I have put some further notes with what the quickstart mode does, but not sure if anything needs to be added.

[hop-dev]

Oh one last thing, the cursor is visible in the top left corner of a couple of the screenshots, not sure if that is an issue

[dedemorton]

Oh one last thing, the cursor is visible in the top left corner of a couple of the screenshots, not sure if that is an issue

That's a little sloppy, but since this PR is getting merged late, I think it's better to get the changes merged and clean up the screens later.

Actually I'll just use my screen capturing tool to edit out the cursors. :-) Simpler than doing new captures.