Add form-associated (custom) element information

### MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/Web_components/Using_custom_elements

### What specific section or headline is this issue about?

_No response_

### What information was incorrect, unhelpful, or incomplete?

Right now on MDN there is no information at all about [form-associated elements](https://html.spec.whatwg.org/dev/custom-elements.html#form-associated-custom-element) like `formAssociated` or `formDisabledCallback` properties. I think adding that would help a lot.

### What did you expect to see?

Information about form-associated elements.

### Do you have any supporting links, references, or citations?

https://html.spec.whatwg.org/dev/custom-elements.html

### Do you have anything more you want to share?

_No response_

### MDN metadata

Page report details

* Folder: `en-us/web/api/web_components/using_custom_elements`
* MDN URL: https://developer.mozilla.org/en-US/docs/Web/API/Web_components/Using_custom_elements
* GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/api/web_components/using_custom_elements/index.md
* Last commit: https://github.com/mdn/content/commit/6d131659eaf1ef1586552f4cead45fa99f73e9c2
* Document last modified: 2023-09-26T15:49:39.000Z