Consider ways to make WIT files more discoverable

[lukewagner]

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.