By default, put the video in an `out` folder #518

This simplifies the .gitignore template by allowing to just ignore the out directory instead of unignoring the .mp4 files in the src folder and also think it's nice to have a dedicated folder.

cc @aVileBroker who came up with the idea

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Approval criteria for this PR:

• Change packages/cli/src/render.tsx so that if you render the video into a folder, and that folder doesn't exist, you create it. The following command should work in the example folder: npx remotion render src/index.tsx react-svg any/nested/folder/video.mp4
• Go through all the templates we support, and instead of rendering to out.mp4, render to out/video.mp4 (adjust the script in package.json). Make a separate PR for each of those templates!
• Also remove the **.mp4 references in the .gitignore of the templates, and add out instead.