Add version document option

[coatless]

Allow for specifying the version of main webR project to be used.

• latest
• v0.3.3
• and so on

This is a toggle to ensure documents work well when the version of R is boosted; but the extension isn't running the latest version.

[coatless]

We probably should add this.

But, the main driver was needing to specify webr-image under the action build recipe for build-rwasm.

      - name: Build WASM R packages
        uses: r-wasm/actions/build-rwasm@v1
        with:
          packages: "."
          repo-path: "_site"
          webr-image: "ghcr.io/r-wasm/webr:v0.3.3"

c.f. r-wasm/actions#22 and r-wasm/rwasm#33