Add S3 permission for current 'main' version.

[d3zd3z]

#210 added permission for the zephyr-lang-rust repo to publish per-PR docs under arn:aws:s3:::builds.zephyrproject.org/zephyr-lang-rust/pr/*. However, the request was also to be able to publish docs for main under arn:aws:s3:::builds.zephyrproject.org/zephyr-lang-rust/main/*., which will have the latest docs of the version in Main.

These main docs should not be removed over time, and will always contain the latest docs for the project.