convert lvgl from submodule to a plain old directory

lib/lvgl/docs/overview/renderers/arm-2d.md

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+# ARM-2D GPU
+
+TODO
+

lib/lvgl/docs/overview/renderers/dma2d.md

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+# DMA2D GPU
+
+TODO
+

lib/lvgl/docs/overview/renderers/index.md

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+
+# Renderers and GPUs
+
+
+```eval_rst
+
+.. toctree::
+   :maxdepth: 2
+
+   sw
+   sdl
+   arm-2d
+   pxp-vglite
+   dma2d
+```
+

lib/lvgl/docs/overview/renderers/pxp-vglite.md

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+# NXP PXP and VGLite GPU
+
+TODO
+

lib/lvgl/docs/overview/renderers/sdl.md

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+# SDL renderer
+
+TODO
+

lib/lvgl/docs/overview/renderers/sw.md

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+# Software renderer
+
+TODO
+