WTO: Write series in JSON repo for "annually" category

Child of #2 (closed)

Story

• As a search engine indexer,
• I want to find series.json files corresponding to the "annually" category,
• In order to index them.

Acceptance criteria

• Each dataset directory MUST contain a series directory
• Each series directory MUST contain a series.json file.
• Each JSON file MUST be validated according to this documentation on the Wiki.