DocGen: changes to correctly merge metadata from tributaries

[Laren-AWS]

This PR adds a few things to let update-mirror correctly merge services, SDKs, and validation metadata from tributaries. With upcoming update-mirror changes, this fully enables the feature that lets tributaries define their own services.yaml, sdks.yaml, and validation.yaml.

• Adds incremental=true to DocGen default so it does loads the default metadata from itself but does not try to load any examples. This defines the "default" DocGen as containing all of the services, SDKs, and validation lists defined in this repo so that it can be used as a basis for merging in tributary examples and additional services, SDKs, and validation.
• Update DocGen.merge to merge validation data from other.

With these changes, update-mirror can then:

1. Load a default DocGen.
2. Merge metadata from a tributary and successfully validate it against the base services, etc., as well as the tributary add-ons.
3. Merge each tributary into the full registry of metadata.

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