[curation] Consider creating back references views per spec

[tidoust]

Via dontcallmedom/webdex#22, whatwg/streams#1369 (comment). Also speced/bikeshed#3155

Reusing code from Webdex, we could build a file per spec that contains, for each term that the spec defines, the list of specs that references it.

This could perhaps be extended to all IDs, regardless of whether they are actual definitions (e.g., headings).