Adds tip about how to get custom dashboards to appear

[benironside]

Fixes #4525 by adding a note about the tag you can add to custom dashboards to make them appear on the security dashboards landing page

[github-actions]

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[mergify]

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

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[benironside]

@elasticmachine run elasticsearch-ci/docs