Add style for non-code placeholder

What does this MR do?

I recently noticed that bullet number 9 in https://docs.gitlab.com/ee/user/group/saml_sso/group_sync.html#configure-azure-ad had incorrect formatting within the UI text due to a placeholder value's formatting of <placeholder>.

I opened !128718 (closed) to correct this and noticed that we don't have any non-code block placeholder formatting style guidance.

This MR proposes to add that guidance, based on the style chosen for the MR. There are other options if the chosen style does not work:

• Use ... and define immediately after i.e. ..., where ... is the definition of the placeholder.
• Use brackets e.g. (placeholder_text)
• Use backticks
• Use double quotes e.g. "placeholder_text"

This MR currently suggests using angled brackets wrapped with a single backtick.

Tagging @marcel.amirault as his feedback on the MR was invaluable.

Related issues

!128718 (closed)

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