Document readonly attribute on <fieldset>

**Spec PR:** https://github.com/whatwg/html/pull/12424
**Spec issue:** https://github.com/whatwg/html/issues/10293
**WPT tests (tentative):** https://github.com/web-platform-tests/wpt/pull/59741

The HTML spec is adding a `readonly` content attribute to ``
that propagates a read-only state to descendant form controls
(parallel to the existing `disabled` attribute).

Pages to update once at least one engine ships:

- `Web/HTML/Element/fieldset` — add `readonly` to the attributes table
alongside `disabled`, with a usage example
- `Web/API/HTMLFieldSetElement` — add the `readOnly` IDL property
- `Web/API/HTMLElement#instance_methods` (custom elements) — add
`formReadonlyCallback` to the form-associated custom element
lifecycle callbacks list, alongside `formDisabledCallback`
- BCD entries for all of the above

Affected descendants are: `` in states that support readonly
(text, search, url, tel, email, password, date, month, week, time,
datetime-local, number), ``, and form-associated custom
elements. Descendants of the first `<legend>` child are exempt.

Filing now so it's tracked; no action needed until shipping.