fix(ci): build workspace packages before demo gen so fhir-gen is on PATH

The new demo-build steps in the docs deploy run `pnpm gen` which calls
`fhir-gen` from @fhir-dsl/cli. pnpm install doesn't auto-build workspace
packages, so the binary symlink resolves to a non-existent file. Add a
`pnpm build` step (root, excludes demo via filter) before the gen step.

Author: awbx

.github/workflows/deploy-docs.yml

@@ -38,6 +38,12 @@ jobs:
       - name: Install dependencies
         run: pnpm install --frozen-lockfile
 
+      - name: Build workspace packages
+        # Builds every @fhir-dsl/* package (excludes apps/his-demo via the
+        # root build script's filter). Required so `fhir-gen` is on PATH for
+        # the demo gen step below.
+        run: pnpm build
+
       - name: Build Docusaurus
         working-directory: apps/docs
         run: pnpm build