ci: drop the dead Docs Preview workflow

The \`Docs Preview\` workflow can't actually deploy a preview today:

  \u2022 The \`build-and-deploy\` job is gated on
    \`head.repo.full_name == github.repository\`, so it skips on every
    PR opened from a fork.
  \u2022 Even on internal-branch PRs, the deploy step is rejected by the
    \`github-pages\` environment, whose custom branch policy only
    allows deployments from \`main\`:

        Branch refs/pull/<N>/merge is not allowed to deploy to
        github-pages due to environment protection rules.

So the workflow burns CI time, posts no preview comment, and leaves a
red FAILURE check on every PR that touches \`docs/**\`. Remove the
workflow file rather than leave a permanently broken check in place.

If a maintainer wants real PR previews back, the right fix is a
follow-up that either widens the \`github-pages\` environment branch
policy or moves the docs-preview deploy to a separate destination
(e.g. Cloudflare Pages, Netlify, or a non-protected \`gh-pages\` push).

Author: dgageot