Document new attribute modifier

[potsky]

Goes along with statamic/cms#9796

Hint for the documentation ;-)

---

id:
blueprint: modifiers
modifier_types:

• markup
  title: Attribute

---

Attribute modifier conditionally renders the parameter that corresponds to the given value, but only when this provided value is not empty.

For example:

---
var: text-pink-500 bg-yellow-200
---

<span{{ view:var | attribute:class }}>Statamic</span>

returns:

<span class="text-pink-500 bg-yellow-200">Statamic</span>

But

---
var: 
---

<span{{ view:var | attribute:class }}>Statamic</span>

will returns:

<span>Statamic</span>

[duncanmcclean]

Documented in ba2ea9e.