DocGen: add complex categories

[Laren-AWS]

Add complex categories as a separate YAML file that defines additional data for a category. The existing category field that uses a simple string is used as a key into the complex category table. The first use case for this is for tributaries that define custom categories and need to specify additional info about them. This also sets us up for expanding categories as needed in the future.

• The category field in the main metadata can now be used as a key rather than a display value.
• Categories contain a display field for their display value and a description field that can be joined with the display value to form a description.
• Categories can also contain default and override specifications for title fields. Defaults are used when metadata does not contain these fields, overrides are always used.

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