Fix typo `inputs.ref` → `matrix.ref`.
This caused an empty string to be assigned to the checkout action's
`ref` input, meaning it used its default value `github.ref`.
Therefore all "versions" of the docs were actually building `main` 😂
Instead, we actually want to checkout the ref from the job's `matrix`.
Extract the "build" part of building & deploying the docs website into a
`build-docs` composite action.
Refactor the workflow to use a matrix job strategy; allowing each branch
to be built in parallel and in isolation.
In a subsequent job, we combine the builds into a single artifact.
