Document `pygame._render`

[damusss]

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Ported the docs from sdl2_video to pygame._render, using the autoapi docs system (I added one line to make it work). The constructor of Renderer and the blit methods are the only one currently referencing things that have not been ported. Tho it won't be like this anymore since the PRs to port them are already up. I can change if needed, but I don't think so as this is an experimental, privated module with a big warning.

EDIT (also explaining things better after discussing with @Starbuck5): I removed _render from index.rst. I also removed render.rst. This means tat the only thing happening with this PR is that the stubs of _render start getting the docs of sdl2_video. When pygame.window was first born, it was still experimental, but it got the documentation, and eventually started getting features while sdl2_video.Window was left behind. _render, written properly in C, is the future replacement of some of the features of sdl2_video.video, which will be "left behind" as well (otherwise, what's even the point), meaning _render, which will become render when it is complete, require docs (and should use the new system). Sooner or later it must have docs. Now, since I'm using the new system, I can add docs without them existing in pyga.me/docs because as long as no render.rst exists, autoapi will not in fact make them exist. The module will stay private. This PR allows for _render to get docs gradually. Otherwise, some time in the future, Renderer, Texture, Image and whatever would have to get their docs all at once. If you are still confused I'm here to explain, or if you have concerns, I'm here for feedback!