Rollup merge of #100927 - andrewpollack:fuchsia-docs-rustup, r=tmandry

Adding new Fuchsia rustup docs... reworking walkthrough

Docs improvements:
* Adding new `rustup` target add for Fuchsia targets
* Reworking walkthrough to show directory building as it happens
* Reworking walkthrough to use `hello_fuchsia_pkg/` directory

cc. `@djkoloski`