Add readme files and screenshots to sample projects (#191)

samples/meta-body-tracking-sample/README.md

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+# Meta Body Tracking Sample
+
+> Note: this project requires Godot 4.3 or later
+
+This is a sample project demonstrating tracking features on Meta headsets. This includes body / hand tracking, which is supported on Quest 2, Quest 3, and Quest Pro;
+as well as face tracking, which is only supported on Quest Pro. For more Meta-specific hand tracking features, check out the [Meta Hand Tracking Sample](https://github.com/GodotVR/godot_openxr_vendors/tree/master/samples/meta-hand-tracking-sample).
+
+# Screenshots
+
+![Screenshot](screenshots/meta_body_tracking_screenshot_01.png)

samples/meta-composition-layers-sample/README.md

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+# Meta Composition Layers Sample
+
+> Note: this project requires Godot 4.3 or later
+
+This is a sample project demonstrating the Meta-specific composition layer features supported by the Godot OpenXR Vendors plugin.
+See the [Meta Composition Layers](https://godotvr.github.io/godot_openxr_vendors/manual/meta/composition_layers.html)
+tutorial doc for a detailed walkthrough on these features.
+
+# Screenshots
+
+![Screenshot 1](screenshots/meta_comp_layers_screenshot_01.png)
+![Screenshot 2](screenshots/meta_comp_layers_screenshot_02.png)

samples/meta-hand-tracking-sample/README.md

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+# Meta Hand Tracking Sample
+
+> Note: this project requires Godot 4.3 or later
+
+This is a sample project demonstrating the Meta-specific hand tracking features supported by the Godot OpenXR Vendors plugin.
+See the [Meta Hand Tracking](https://godotvr.github.io/godot_openxr_vendors/manual/meta/hand_tracking.html) tutorial doc for
+a detailed walkthrough of these features.
+
+# Screenshots
+
+![Screenshot](screenshots/meta_hand_tracking_screenshot_01.png)

samples/meta-passthrough-sample/README.md

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+# Meta Passthrough Sample
+
+> Note: this project requires Godot 4.3 or later
+
+This is a sample project demonstrating the Meta-specific passthrough features supported by the Godot OpenXR Vendors plugin.
+See the [Meta Passthrough](https://godotvr.github.io/godot_openxr_vendors/manual/meta/passthrough.html) tutorial doc for
+a detailed walkthrough of these features.
+
+# Screenshots
+
+![Screenshot 1](screenshots/meta_passthrough_screenshot_01.png)
+![Screenshot 2](screenshots/meta_passthrough_screenshot_02.png)

samples/meta-scene-sample/README.md

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+# Meta Scene Sample
+
+> Note: this project requires Godot 4.3 or later
+
+This is a sample project demonstrating the Meta scene and spatial anchor features supported by the Godot OpenXR Vendors plugin.
+See the [Meta Scene Manager](https://godotvr.github.io/godot_openxr_vendors/manual/meta/scene_manager.html)
+or [Meta Spatial Anchors](https://godotvr.github.io/godot_openxr_vendors/manual/meta/spatial_anchors.html)
+tutorial docs for detailed walkthroughs of these features.
+
+# Screenshots
+
+![Screenshot](screenshots/meta_sceen_screenshot_01.png)