CI doesn't fail if examples don't match the schema sometimes

The bug fixed by https://github.com/matrix-org/matrix-doc/pull/1879 wasn't caught by CI because the top level schema had `additionalProperties`. We might want to consider a way to detect a scenario where someone accidentally does this and erroring on it.

One potential solution is having the checker realize that `m.room_versions` (in this case) is nested in the example but not nested by the schema. Another option is to realize that `additionalProperties` rarely turns up on the top level object and complaining about that.

We could also introduce another extension (at least I think it would be an extension...) to apply a simple regex to additional properties. In this particular case, `/.*\./` would have caught the error by enforcing properties be moderately namespaced.

Start by reviewing PR 1879 and the schema/example checker behavior around nested m.room_versions and top-level additionalProperties. Compare the proposed detection strategies and define which cases CI must reject; done means accidental schema mismatches reliably fail validation.

Written by the indexing model from the issue text.