Fix broken Confidential Token repo link on Privacy page

[kaankacar]

Closes #2914

What

• docs/build/apps/privacy.mdx: replaced the dead link to OpenZeppelin's deleted feat/confidential-verifier-ultrahonk branch with the confidential token package at packages/confidential on main. Added links to the deployment guide and examples and to the design overview.
  • The demo bullet now says "(developer preview)". The demo still targets the deleted branch.
  • "Implementation on Stellar is in progress." now points to the stellar-contracts library.
  • The caution is unchanged. The contracts are still unaudited.
• docs/tools/openzeppelin-contracts.mdx: no change yet.

Verification

• New external links, all HTTP 200. The GitHub paths also resolve through the contents API on main.
  • https://github.com/OpenZeppelin/stellar-contracts
  • https://github.com/OpenZeppelin/stellar-contracts/tree/main/packages/confidential
  • https://github.com/OpenZeppelin/stellar-contracts/tree/main/examples/confidential
  • https://github.com/OpenZeppelin/stellar-contracts/blob/main/packages/confidential/docs/overview.md
• The old link returns 404. The branch is gone from git ls-remote.
• Release and audit facts come from releases and the audits folder. The latest tag is v0.8.0-rc.3, without packages/confidential. The latest audit report covers v0.7.0.
• pnpm check:mdx passes. I did not run pnpm build locally. The change adds no internal links or anchors, and CI runs the full build.

Notes

• Downstream: stellar/ecosystem-resources picks this up on the triage bot's next daily sync after merge.
• Draft until OpenZeppelin tags the stable release. Then links get pinned from main to the tag and the caution/"Coming Soon" updates are applied in this same PR.
• Draft PR 🔒 Refresh Privacy on Stellar guide #2673 edits the same lines. One of the two PRs will need a rebase.

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