Automatically generate extractor/loader documentation from `discovery.yml`
The docs at https://meltano.com/plugins/extractors/ and https://meltano.com/plugins/loaders/ are currently maintained separately from https://gitlab.com/meltano/meltano/blob/master/src/meltano/core/bundle/discovery.yml, which can lead to inconsistency between the docs and discovery.yml, and between docs for different plugins.