Rollup merge of #137868 - taiki-e:powerpcspe-doc, r=workingjubilee

Add minimal platform support documentation for powerpc-unknown-linux-gnuspe

Per rust-lang/rust#137860 (comment), add minimal platform support documentation, including a brief summary and links to more detailed information about this target.

The added documentation is minimal. This is somewhat similar to [powerpc-unknown-openbsd, which also has no target maintainer](https://doc.rust-lang.org/nightly/rustc/platform-support/powerpc-unknown-openbsd.html). The rest of the template is left to target maintainers.

I also updated powerpc-unknown-linux-muslspe platform support documentation and added link to powerpc-unknown-linux-gnuspe platform support documentation.

cc ```@glaubitz```
cc ```@BKPepe```
r? workingjubilee

```@rustbot``` label +O-PowerPC

Author: compiler-errors