Update the EMS section in the air-gapped documentation

[jsanz]

Related to elastic/kibana#175551

This is a small edit in the EMS section to link to the full document and explain better the three options available to users regarding the product in air-gapped environments.

cc. @nreese

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[mergify]

This pull request does not have a backport label. Could you fix it @jsanz? 🙏
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 a lot for adding this @jsanz. I've added a label so that this backports into the current, 8.12 docs.

[jsanz]

Thanks, @kilfoyle; I don't have permission to add labels or merge, so please could you merge it?

[kilfoyle]

@jsanz, Sure, I'll be happy to merge and backport it. Just let me know when it's ready (as I assume you'll want to add in Nathan's suggestion).

[jsanz]

@jsanz, Sure, I'll be happy to merge and backport it. Just let me know when it's ready (as I assume you'll want to add in Nathan's suggestion).

PR is ready, @kilfoyle; thanks for the support!

[nreese]

LGTM. Thanks for making these changes.