Fleet Server Scaling should contain the note about max 500 policies

[philippkahr]

Hi,

Policy scaling recommendations
edit
A single instance of Fleet supports a maximum of 500 Elastic Agent policies. If more policies are configured, UI performance might be impacted.

This information is burried way too deep and while yes it makes sense to have it at the agent policies, I honestly never scrolled down that far, because it's just a 101 of how to create a policy. We have a fleet server scaling doc. I think the 500 policies note should be part of that page.

[mergify]

This pull request does not have a backport label. Could you fix it @philippkahr? 🙏
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.

[kilfoyle]

LGTM! Thanks for adding this @philippkahr

I took the liberty of pushing a small commit that should fix the failing docs CI check.

[kilfoyle]

P.S., I hope you don't mind that I went ahead and merged this. :-)