Document `Cross-Origin-Opener-Policy-Report-Only` HTTP header

### Proposal

The WHATWG HTML Standard includes an HTTP field `Cross-Origin-Opener-Policy-Report-Only`, which has been introduced into the standard since November 2020 (merge time of whatwg/html#5518).

### Browser support

Chromium implementation bug (marked as fixed in Dec 2020): https://issues.chromium.org/issues/40121546
I'm not sure what version of Google Chrome this would link to.

### Tasks

- [ ] Add https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Cross-Origin-Opener-Policy-Report-Only
- [ ] Update https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Cross-Origin-Opener-Policy
- [ ] Update https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers

### Dependencies

- [ ] BCD entry at https://github.com/mdn/browser-compat-data/tree/main/http/headers

### Additional information

- WHATWG HTML Standard: https://html.spec.whatwg.org/multipage/browsers.html#cross-origin-opener-policy-report-only
- Chromium Intent-to-Ship: https://groups.google.com/a/chromium.org/g/blink-dev/c/0fvyrO1-p_o
- Explainer document (also linked from Intent-to-Ship page): https://github.com/camillelamy/explainers/blob/main/coop_reporting.md
- Original PR introducing field to WHATWG: https://github.com/whatwg/html/pull/5518

### Are you willing to support this work?

I'm new to contributing to MDN, but I'm willing to write a PR that adds a page for this