docs(common): remove h1s from API docs

Only one h1 is allowed per document, and this is provided by the template.
So we cannot have any h1 tags (or `#` markdown shorthand) in any API docs.

Closes #16193

packages/common/src/directives/ng_component_outlet.ts

@@ -49,7 +49,7 @@ import {ComponentFactoryResolver, ComponentRef, Directive, Injector, Input, NgMo
  *                                   ngModuleFactory: moduleFactory;">
  * </ng-container>
  * ```
- * # Example
+ * ## Example
  *
  * {@example common/ngComponentOutlet/ts/module.ts region='SimpleExample'}
  *

packages/common/src/directives/ng_if.ts

@@ -17,13 +17,13 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef} from '
  *  - `then` template is the inline template of `ngIf` unless bound to a different value.
  *  - `else` template is blank unless it is bound.
  *
- * # Most common usage
+ * ## Most common usage
  *
  * The most common usage of the `ngIf` directive is to conditionally show the inline template as
  * seen in this example:
  * {@example common/ngIf/ts/module.ts region='NgIfSimple'}
  *
- * # Showing an alternative template using `else`
+ * ## Showing an alternative template using `else`
  *
  * If it is necessary to display a template when the `expression` is falsy use the `else` template
  * binding as shown. Note that the `else` binding points to a `<ng-template>` labeled `#elseBlock`.
@@ -32,7 +32,7 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef} from '
  *
  * {@example common/ngIf/ts/module.ts region='NgIfElse'}
  *
- * # Using non-inlined `then` template
+ * ## Using non-inlined `then` template
  *
  * Usually the `then` template is the inlined template of the `ngIf`, but it can be changed using
  * a binding (just like `else`). Because `then` and `else` are bindings, the template references can
@@ -40,7 +40,7 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef} from '
  *
  * {@example common/ngIf/ts/module.ts region='NgIfThenElse'}
  *
- * # Storing conditional result in a variable
+ * ## Storing conditional result in a variable
  *
  * A common pattern is that we need to show a set of properties from the same object. If the
  * object is undefined, then we have to use the safe-traversal-operator `?.` to guard against

packages/common/src/directives/ng_template_outlet.ts

@@ -26,7 +26,7 @@ import {Directive, EmbeddedViewRef, Input, OnChanges, SimpleChanges, TemplateRef
  *
  * Note: using the key `$implicit` in the context object will set it's value as default.
  *
- * # Example
+ * ## Example
  *
  * {@example common/ngTemplateOutlet/ts/module.ts region='NgTemplateOutlet'}
  *