Migrate to MDN metadata.json

[saschanaz]

MDN has an API endpoint that gives summaries for each document: https://developer.mozilla.org/en-US/metadata.json

The data looks sufficient for this repository, and we can save a submodule (and prevent pain like #2137, also makes #2078 redundant)

[saschanaz]

We should store the metadata in inputfiles/ so that the build can be deterministic, and make an npm command to update it, and use it in the workflow.

[Bashamega]

Can I work on this?

[saschanaz]

Sure!

[Bashamega]

Thanks

[Bashamega]

Would you like it to write to the comments.json file, or create a new file? Also, if it creates a new file, will it be in our format or in the MDN format, requiring a parser for it?

[Bashamega]

How should I do it @saschanaz