Clean up GeneratorBackend.

[kallentu]

• Merge GeneratorBackend and GeneratorBackendBase. Removed GeneratorBackendBase essentially.
• Moved all the documentation.
• HtmlGeneratorBackend now extends GeneratorBackend
• Removed generateConstant and generateTopLevelConstant. I really don't think we need this indirection.

In prep for #3667. Thought it'd be good to send this out on its own.

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[srawlins]

Nice! Yes, this is definitely good as its own PR. Thanks!