docs: fix release-3.0 Metal proving notes

[peter941221]

This updates the release-3.0 documentation to match the prover selection code on that branch.

Failure mechanism:
The release docs still say Metal proving is enabled by default on Apple Silicon, but both prover selection paths on release-3.0 fall back to CPU unless CUDA is enabled:

• risc0/circuit/rv32im/src/prove/mod.rs
• risc0/circuit/recursion/src/prove/mod.rs

Semantic change:

• replace the deprecated feature-table wording in the top-level README and zkVM docs
• update the local proving docs to state that release-3.0 does not enable Metal proving by default on Apple Silicon and falls back to CPU in this release line

Files changed:

• README.md
• risc0/zkvm/src/lib.rs
• risc0/zkvm/README.md
• website/api/generating-proofs/local-proving.md

Testing:

• inspected the release-3.0 diff for scope and wording only
• did not run a docs build

[vercel]

@peter941221 is attempting to deploy a commit to the Boundless Team on Vercel.

A member of the Team first needs to authorize it.

[peter941221]

The current failing status here is Vercel, with Authorization required to deploy. That looks like preview-deploy authorization rather than a code failure in the patch. If Vercel is expected for this PR, could someone with access authorize or rerun it? If not, please let me know whether this check can be ignored for fork PRs.