Build the docs with pmndrs/docs@v4

[abernier]

One line: the reusable workflow moves from @v3/@v2 to @v4.

pmndrs/docs v4 drops the Docker image. The workflow now builds through npx @pmndrs/docs@latest instead of docker run ghcr.io/pmndrs/docs.

Nothing else changes for this repository: the same inputs, the same environment variables, the same GitHub Pages artifact. The only removed input is docker_tag, which this workflow never passed.

The old tag stays where it is, so this repository keeps building on the Docker path until this merges.

Note

These workflows only trigger on a push to the default branch, so this pull request runs no build of its own — the first real run happens after the merge.