Add a data source page for FERC CID

[krivard]

Overview

Most of our primary data sources have their own data source page, explaining the background on the source, notable irregularities, and any transformations we apply. We use the FERC CID in at least one published table (core_ferc__entity_companies) so we should probably include it in our sources list.

Success Criteria

How will we know that we're done?

• A page exists at https://docs.catalyst.coop/pudl/en/latest/data_sources/ferccid.html with content according to our usual data source format & quality standards