Rollup merge of #132477 - Noratrieb:illumos-oxide, r=workingjubilee

Add illumos target documentation

Fixes https://github.com/rust-lang/rust/pull/130132#issuecomment-2339055221

`@jclulow` `@pfmooney` I'm adding you as requested.

The page is very barebones (as I do not know illumos well) and could use some improvements (for example in the "Cross-compilation toolchains and C code" section).
Feel free to suggest improvements (or rewrite it from scratch) if you find something.