[Meta] Make navigation to outdated hashes in the spec possible

[tomayac]

In a Twitter conversation, @zcorpan and I discussed HTML elements that could have been, and Simon brought up <menuitem>, which was removed in #2730. From there, I followed the link to crbug/87553, which then led me to https://html.spec.whatwg.org/multipage/forms.html#context-menus, but the ID #context-menus no longer exists.

In such cases, it would be great if a note appeared that guided the reader to past snapshots (https://html.spec.whatwg.org/commit-snapshots/942a9d5b9c0b50bf5bca275e866bb2319ebdca0c/#context-menus in the concrete case). Ideally, the note even pointed the reader at the latest snapshot where the ID still appeared.