Improve ergonomics of Rust examples, re-enable use of published packages

[vados-cosmonic]

This issue arguably shouldn't go here, but currently UX issues in upstream tooling is making the Rust

The goal is to revert this reverted functionality -- we want to be able to use published WIT packages but there are a few issues stopping us:

• cargo-component does not configure/patch where wkg looks for deps (the target kind) when configured in Cargo.toml (i.e. we should be able to specify that docs is a known package w/ a known registry)
• cargo-component currently does not properly resolve targets w/ multiple worlds even if wkg is configured (via it's global config file)
• wkg should be able to use local fconfig

There are at least a few ways to solve this issue:

1. Update cargo-component to perform the patching of registries for namespaces before using wasm-pkg-tools functionality
2. Update wkg to prefer local config files and include one in the tutorial code
3. Update the guide to get users to configure wkg global config before attempting usage.

A somewhat hidden requirement/sidequest to this issue is that we need more cargo-component/wasm-pkg-tools maintainers. The current functionality is fantastic for these tools, but the UX needs to be polished just a bit.