feat: add UNTP public review banner and per-page feedback link

This MR implements the formal UNTP public review affordance described in #654 (closed), generalised so any version (not just the WIP) can be flagged as currently in public review.

Closes #654 (closed)

What's included

PublicReviewBanner

Shown on the version currently in formal public review. Contains a mailto: link that opens the user's mail client with a fillable email template addressed to untp-public-review@gaggle.email, with subject UNTP feedback on <page title> and a body pre-populated with the page URL plus blank fields for submitter name, organisation, country, and proposed changes.

WipDuringReviewBanner

Shown on WIP/current pages while a non-WIP version is in review. Tells the reader that feedback received during the review period is being incorporated, with a link to the corresponding page on the in-review version.

Disclaimer copy

Updated from "is not ready for implementation" to "is suitable for pre-production pilot implementations" (current and v0.6.0).

Unmaintained banner

Title Outdated → Unmaintained; default message wording aligned ("You're viewing an unmaintained version of …").

Notes

• No automated tests in this MR. The website has no Jest setup and no tsconfig.json, and introducing both is meaningfully larger than this feature. Logic is factored into pure modules (buildMailto, etc..) so unit tests can be added as a follow-up. A separate issue covering test infrastructure setup will be raised.
• 0.7.0 snapshot hasn't been created. Some of the screenshots above show v0.7.0. That version is not part of this MR. It was only created locally to demonstrate the WipDuringReviewBanner (which only renders when a non-WIP version is in public review). A subsequent MR will be raised for the actual v0.7.0 release.