docs: add IMPORTANT prefix to pipe usage warning and

Adds an IMPORTANT label to emphasize the guidance about avoiding impure
pipes due to potential performance impact.

Author: MeAkib

adev/src/content/guide/templates/pipes.md

@@ -271,7 +271,7 @@ export class MyCustomTransformationPipe implements PipeTransform {
 
 When you want a pipe to detect changes within arrays or objects, it must be marked as an impure function by passing the `pure` flag with a value of `false`.
 
-Avoid creating impure pipes unless absolutely necessary, as they can incur a significant performance penalty if used without care.
+IMPORTANT: Avoid creating impure pipes unless absolutely necessary, as they can incur a significant performance penalty if used without care.
 
 ```angular-ts
 import { Pipe, PipeTransform } from '@angular/core';