[DOCS-13661] Document triple curly brace syntax for raw/unencoded output

[estherk15]

What does this PR do? What is the motivation?

Fixes DOCS-13661

Expands the "Character escape" section on the Monitor Notification Variables page to document the triple curly brace syntax ({{{variable}}}) for raw, unencoded output.

Changes include:

• Clarifies that {{variable}} HTML-encodes output by default (with concrete examples of what gets encoded)
• Adds a comparison table showing {{variable}} vs {{{variable}}}
• Adds a practical example using {{{check_message}}}
• Explains the real-world impact: HTTP Check monitor URLs with query parameters break when & is HTML-encoded

This behavior has been a recurring support topic (MNTS-89137, MNTS-89854, SFDC 00012692).

Merge instructions

Merge readiness:

• Ready for merge

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/esther/docs-13661-triple-curly-brace-variable/monitors/notify/variables

[maycmlee]

Does this mean if you put this in the notification message template:

This & that

In the actual notification received, you'd see:

This & that

If so, maybe this could make it clearer:

Suggested change

	Variable content is HTML-encoded by default. For example, special characters like `&` and `'` are converted to `&` and `'` in the rendered output. To output raw, unencoded content, use triple curly braces instead of double curly braces:
	Variable content is HTML-encoded by default. For example, if a notification message contains special characters like `&` and `'`, the characters are converted to `&` and `'` in the rendered output. To output raw, unencoded content, use triple curly braces instead of double curly braces: