Pin pnpm version in CI

The workflows asked for `version: latest`, so CI silently moved to pnpm 11
while local development stays on 10.18.3. pnpm 11 removed the
`onlyBuiltDependencies` setting in favor of `allowBuilds`, so the approval
list in docs/pnpm-workspace.yaml no longer registered and every install
failed with ERR_PNPM_IGNORED_BUILDS for @tailwindcss/oxide, sharp, and
unrs-resolver.

Pinning to the version used locally keeps CI reproducible and lets it
verify what developers actually run. pnpm 11 also requires Node >= 22.13,
so adopting it would mean upgrading local toolchains first.

Author: ndbroadbent

.github/workflows/deploy-docs.yml

@@ -43,7 +43,7 @@ jobs:
       - name: Setup pnpm
         uses: pnpm/action-setup@v4
         with:
-          version: latest
+          version: 10.18.3
 
       - name: Enable pnpm cache
         uses: actions/setup-node@v4

.github/workflows/test.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Setup pnpm
         uses: pnpm/action-setup@v4
         with:
-          version: latest
+          version: 10.18.3
 
       - name: Enable pnpm cache
         uses: actions/setup-node@v4
@@ -112,7 +112,7 @@ jobs:
       - name: Setup pnpm
         uses: pnpm/action-setup@v4
         with:
-          version: latest
+          version: 10.18.3
 
       - name: Enable pnpm cache (docs)
         uses: actions/setup-node@v4