Move documentation into docs/ dir

[yoshuawuyts]

This PR centralizes all of the WASI documentation files in the docs/ dir. This includes sources from:

• The project root dir
• The proposals/ dir
• The main README.md file

The goal with this is make it easier to find what people are looking for. As well as setting us up for further iteration on the documents themselves in the future. We might potentially even deputize specific owners or editors for the docs in the future other than the WASI SG chairs.

One notable change in this PR is that it moves the Contributing.md file from the project root to the docs/ directory. But according to GitHub that is a valid place to put it, and will be detected by the GitHub UI.

The other notable change is that this moves Proposals.md out of the project root. I'm marking this PR as a draft since there are a number of external resources that link to Proposals.md on main. I want to file draft PRs against all known references in both the WebAssembly and BytecodeAlliance orgs before marking this PR as ready to review.

Thanks!

[yoshuawuyts]

Had an offline conversation with @ricochet who made the case for keeping Contributing.md in the repo root, alongside the README.md file. On the condition that we do make it all-caps to fit the predominant style.

I've agreed to this and will make the change.