create bootstrap guide for direnv setup

[jdx]

direnv has 2 commands source_url and fetchurl. I haven't used these before but they will download files from the internet if you match them to a sha in your envrc config.

I haven't fully thought out how this would work, but I have an idea that we could make the direnv setup not involve installing rtx first. You could just copy a snippet into ~/.config/direnv/lib/use_rtx.sh that is capable of fetching rtx if it doesn't exist. It would probably need to fetch assets from github releases, or maybe we can put them on rtx.jdxcode.com.

[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).