Migrate WASI 0.1 docs from legacy/ dir to wasi-0.1 branch

[yoshuawuyts]

This PR moves the content of the legacy/ directory to a dedicated wasi-0.1 branch. This should make the WASI 0.1 archive easier to find and link to for those who want to, while keeping the main branch focused on the development of upcoming WASI versions. This should create a better experience overall for people who care exclusively about WASI 0.1.

While this PR only affects the WASI 0.1 archive, the plan is to repeat this once more with the introduction of WASI 0.3. Just like we're doing here, we would create a new branch labeled wasi-0.2 and then use that as the archive for the existing WASI 0.2 interfaces and definitions. And with the introduction of WASI 1.0, we would do this one last time.

I've also gone ahead and setup branch protection rules for the WASI 0.1 branch to ensure it cannot just be deleted or force pushed to. That way we're treating it as a long-lasting archive branch, and not just something that is at risk of accidental deletion. For WASI 0.2 and beyond specifically, this also puts us in a position where we can consider publishing additional point releases should there ever be a need to.

I've also gone ahead and searched for any links to the legacy/ directory on GitHub, but from what I can tell all references are either historical or are forks of this repository. In a follow-up PR I'd like to see whether we can re-format the README a little, and see if we can start listing historical WASI versions under a dedicated heading to make it easier to find at a glance.

Thanks!