Add escaping variable section.

[blakerouse]

Adds a section to the elastic-agent-variable-substitution.asciidoc document to explain the new variable escaping.

See: elastic/elastic-agent#5035

Closes elastic/elastic-agent#2177

[github-actions]

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[mergify]

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