docs: Add @defer API reference documentation (#52336)

This adds the initial API reference documentation for the new defer blocks.

PR Close #52336

aio/content/blocks/core/defer.md

-Placeholder content
+A type of [block](api/core/defer) that can be used to defer load the JavaScript for components, directives and pipes used inside a component template.
+
+@syntax
+
+```html
+@defer ( on <trigger>; when <condition>; prefetch on <trigger>; prefetch when <condition> ) {
+  <!-- deferred template fragment -->
+  <calendar-cmp />
+} @placeholder ( minimum? <duration> ) {
+  <!-- placeholder template fragment -->
+  <p>Placeholder</p>
+} @loading ( minimum? <duration>; after? <duration> ) {
+  <!-- loading template fragment -->
+  <img alt="loading image" src="loading.gif" />
+} @error {
+  <!-- error template fragment -->
+  <p>An loading error occured</p>
+}
+```
+
+@description
+
+<h3>Blocks</h3>
+
+Supported sections of a defer block. Note: only the @defer block template fragment is deferred loaded. The remaining optional blocks are eagerly loaded.
+
+| block       | Description |
+| ----------- | ----------- |
+| `@defer`      | The defer loaded block of content |
+| `@placeholder` | Content shown prior to defer loading (Optional)|
+| `@loading` | Content shown during defer loading (Optional) |
+| `@error` | Content shown when defer loading errors occur (Optional) |
+
+<h3>Triggers</h3>
+
+Triggers provide conditions for when defer loading occurs. Some allow a template reference variable as an optional parameter. Separate multiple triggers with a semicolon.
+
+| trigger     | Triggers... | 
+| ----------- | ----------- |
+| `on idle`   | when the browser reports idle state (default) |
+| `on viewport(<elementRef>?)`   | when the element enters the viewport |
+| `on interaction(<elementRef>?)`   | when clicked, touched, or focused |
+| `on hover(<elementRef>?)`   | when element has been hovered |
+| `on immediate`   | when the page finishes rendering |
+| `on timer(<duration>)`   | after a specific timeout |
+| `when <condition>`   | on a custom condition |
+
+<h2>Prefetch</h2>
+
+Triggers prefetching of the defer block used in the `@defer` parameters. Separate multiple prefetch triggers with a semicolon.
+
+```html
+@defer (prefetch on <trigger>; prefetch when <condition>) {
+  <!-- deferred template fragment -->
+}
+```
+