Consider ways to make WIT files more discoverable

I got a bit of feedback from someone who wanted to jump in and check out the wasi:http@0.3.0 WITs that they weren't easy to find. I think perhaps the following tweaks would have helped them, but happy to discuss alternatives:

1. Inside each specifications/R release directory, add a wit subdirectory that contains a subdirectory for each proposal X that contains a copy of the WIT files from proposals/X/wit for that release (so, e.g., there'd be a specifications/wasi-0.3.0/wit/http/types.wit).
2. Update the existing per-proposal specifications/R/X/Overview.md files to say "if you want to see the most-recent draft state of this proposal, see [proposals/X] (link)" so that it's easy to hop from a versioned snapshot to the live draft.
3. Add links in the README.mds of all the existing archived wasi-X proposal repos to a corresponding versioned snapshot in specifications/R/X and the live draft in proposals/X (since these archived repos still tend to show up in docs and search results).

I know that this introduces some duplication within the repo, but if we never allow mutation of WIT files in specifications (only the initial copy), maybe it's ok? Happy to discuss.

Review the existing specifications/R release directories, per-proposal Overview.md files, proposals/X/wit contents, and README.md files in archived wasi-X proposal repositories. First compare the current snapshot and draft layouts, then confirm the intended release/proposal mappings. Done means the proposed WIT copies and navigation links are consistently present without mutating files under specifications.