Point the error-message URL at the live docs site

[n1ckyb]

The documentation site is now live:

https://buchochelliq-labs.github.io/intentumdiff-docs/

Source: intentumdiff-docs. Every
Python example is executed in CI against a real installed wheel and compared with its
documented output; every link is checked with lychee.

Two follow-ups this creates:

• Point the plugin-metadata error URL at the docs site rather than the interim
  PLUGIN_GUIDE.md link on GitHub. This is the "proper URL" the error message was always
  supposed to have, replacing the dead docs.intentumdiff.dev
• Link the site from the PyPI project page and both marketplace listings

Note the docs verify job is currently red on purpose: it asserts the published wheel
produces what the docs claim, and 0.0.1b1 still carries the first-party trust bug. It turns
green when 0.0.2b1 ships the fix. It is a live canary on the published artefact, so it should
not be silenced.