☂️ Introduce docs for new "Custom graphics" section

[parlough]

We recently introduced a new "Custom graphics" section under "Design & theming". It is mostly empty outside of using fragment shaders. We should begin expanding this category.

The following serves as an in-progress list of potential topics to discuss. Feel free to add or adjust. Not all entries are necessarily meant for their own page.

• [ ] Painting a custom widget
• [ ] Load and animate 3D assets
• [ ] Manipulate and transform a render object
• [ ] Manually render graphics

[szmarczak]

FWIW you may want to update this faq section: https://docs.flutter.dev/resources/faq#can-i-build-3d-opengl-apps-with-flutter