v1.4.0: Enable the generation of Markdown docs for Glean metrics and pings (#89)

* Add a second data review to the core.yaml test file

* Add the Markdown outputter and template

This allows to produce human-readable docs
from Glean registry files.

* Add the Kotlin outputter to the translate unit

* Add test coverage for the markdown outputter

* Make flake8 and black happy

* Bonus: remove the double definition of 'description'

* Don't break the table for multiline extra keys description

* Fetch ping information for custom pings from pings.yaml

* Add a disclaimer to the Markdown template

* Bonus: mention that jinja templates are not auto generated

* Remove dead code