This pull request implements the following documentation changes:

- [x] Copy existing language settings docs from old docs repo
- [x] Add new pages for Zig, Haskell, Gleam, Deno and PureScript
- [x] Add `rust-analyzer` target directory section to Rust language page

Release Notes:

- Added initial language settings documentation
([#4264](https://github.com/zed-industries/zed/issues/4264)).