create bootstrap guide for direnv setup about mise

[jdx]

I've put the assets onto rtx.jdxcode.com, so you can do:

$ curl https://rtx.jdxcode.com/rtx-latest-macos-arm64 > rtx
$ chmod +x rtx
$ ./rtx -v

I don't know why I bothered using tar.gz (either tar or gz) for publishing the runtimes before. I think curl/netlify should automatically gzip the contents anways (I may need to configure something in netlify for this to work though, didn't seem to in my basic test).