Add info about experiments_column_type to Jetstream Config data source docs

[mikewilli]

experiments_column_type is a field configured in metric-hub to tell jetstream/mozanalysis more info about how to build queries. It is a string field that has a few valid options -- the right option for the data source depends on the structure of the data.

Options as of writing (from here):

EXPERIMENT_COLUMN_TYPES = (None, "simple", "native", "glean")

For more context to whoever ends up writing this, here are the changes made to queries based on the value of this field:

• https://github.com/mozilla/mozanalysis/blob/main/src/mozanalysis/metrics/__init__.py#L125
• https://github.com/mozilla/mozanalysis/blob/main/src/mozanalysis/metrics/__init__.py#L269

[mikewilli]

Also, I think this info should go in this section.