Mark potentially outdated pages with a WIP warning

getting_started/introduction/godot_design_philosophy.rst

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 .. _doc_godot_design_philosophy:
 
 Godot's design philosophy

getting_started/introduction/index.rst

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 .. Intention: provide the necessary information to make the most of the getting
    started series, answering questions like "do I want to learn Godot?", "how
    does it look and feel?", "how does it work?", and "how do I best learn it?".

getting_started/introduction/introduction_to_godot.rst

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 .. _doc_introduction_to_godot:
 
 Introduction to Godot

getting_started/introduction/key_concepts_overview.rst

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 .. Intention: introduce only a handful of key concepts and avoid a big cognitive
    load. Readers will then be reminded of the concepts further in the getting
    started series, reinforcing their learning.

getting_started/introduction/learning_new_features.rst

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 .. Keep this page short and sweet! We want users to read it to the end, so they
    know where to find information, how to get help, and how to maximize chances
    of getting answers.