Update METADATA documentation and add a check to ensure its complete

[davelab6]

As mentioned in fonttools/fontbakery#3419 and seen at

https://github.com/google/fonts/blob/main/ofl/notosansvai/METADATA.pb#L19-L30

we have new METADATA keys that need to be documented here. This also suggests we could add a Github Action to this repo that download the main /google/fonts repo and greps all the METADATA files for their keys, deduplicates and sorts the list, and then fails if that list is different to the list of documented keys in our docs here.

[felipesanches]

yeah... we need the METADATA.pb (and designers catalog info.pb) schema to be kept in a public repo so we keep our tools in sync.

[davelab6]

That's not going to happen so we need to work around that fact...

[felipesanches]

Ok.