v2: Remove Deprecated Plugin Callbacks

[Dalgona]

Completely replace the following callbacks with newer ones:

• build_started/2
• reading_pages/1
• reading_posts/1
• reading_templates/1
• processing_page/1
• processing_post/1
• processing_template/1
• processed_page/1
• processed_post/1
• processed_template/1
• processed_list/1
• processed_pages/1
• processed_posts/1
• rendering_fragment/2
• rendered_fragment/1
• rendered_page/1
• wrote_file/1
• build_succeeded/2
• build_failed/3
• `finalizing/2

And make implements/0 callback return a keyword list.

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• Update codes
• Update tests
• Update docs