docs: make structural directives guide generic (#44895)

tweak the current structural directives guide (currently mainly targeted
at the creation of custom structural directives) so that is more generic
and a point of reference for structural directives in general

this also includes the re-addition of the one-per-element section
removed in PR #40015

resolves #44786

PR Close #44895

packages/common/src/directives/ng_for_of.ts

@@ -67,11 +67,11 @@ export class NgForOfContext<T, U extends NgIterable<T> = NgIterable<T>> {
  * context according to its lexical position.
  *
  * When using the shorthand syntax, Angular allows only [one structural directive
- * on an element](guide/built-in-directives#one-per-element).
+ * on an element](guide/structural-directives#one-per-element).
  * If you want to iterate conditionally, for example,
  * put the `*ngIf` on a container element that wraps the `*ngFor` element.
  * For futher discussion, see
- * [Structural Directives](guide/built-in-directives#one-per-element).
+ * [Structural Directives](guide/structural-directives#one-per-element).
  *
  * @usageNotes
  *

packages/common/src/directives/ng_if.ts

@@ -143,7 +143,7 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef, ɵstri
  *
  * The presence of the implicit template object has implications for the nesting of
  * structural directives. For more on this subject, see
- * [Structural Directives](https://angular.io/guide/built-in-directives#one-per-element).
+ * [Structural Directives](guide/structural-directives#one-per-element).
  *
  * @ngModule CommonModule
  * @publicApi