docs(hgroup): adds related note from a different page (#30856)

* docs(hgroup): adds related note from a different page

I was reading through this page:
https://developer.mozilla.org/en-US/docs/Web/HTML/Content_categories

when I came across this note:

Note: The <hgroup> element is not recommended as it does not work properly with assistive technologies. It was removed from the W3C HTML specification prior to HTML 5 being finalized, but is still part of the WHATWG specification and is at least partially supported by most browsers.

I believe it's important to include this note in <hgroup>'s page because as it stands right now it might mislead developers into using it, thinking that it has no drawbacks.

Someone might use <hgroup> instead of a <div> due to preference, so they can use it in a css selector or to create a reusable pre-styled container and unknowingly make their website less acessible.

* refactor(hgroup): moves note to the top

Following `<optgroup>`'s example, I think it would be better for such an important note to be at the top

* refactor(hgroup): removes self-referencing anchor tag

* Update files/en-us/web/html/element/hgroup/index.md

---------

Co-authored-by: Estelle Weyl <[email protected]>

files/en-us/web/html/element/hgroup/index.md

@@ -11,6 +11,8 @@ The **`<hgroup>`** [HTML](/en-US/docs/Web/HTML) element represents a heading and
 
 {{EmbedInteractiveExample("pages/tabbed/hgroup.html", "tabbed-standard")}}
 
+> **Note:** The `<hgroup>` element is not recommended as it does not work properly with assistive technologies. It was removed from the W3C HTML specification but is still part of the WHATWG specification and is at least partially supported by most browsers.
+
 ## Attributes
 
 This element only includes the [global attributes](/en-US/docs/Web/HTML/Global_attributes).