Remove docs build since Vercel is doing it, and don't run code tests …

…on docs-only changes. (langchain-ai#2542)

* Check a dev build of the docs, not a prod optimized build.

The `build` CI step was running a `yarn build` for everything, including docs. This is extremely slow (~15-17min) and unnecessary -- the unit tests already do a `yarn build` for the examples and for langchain itself, so the end result was duplication of work and an unnecessary (slow!) production build of the docs.

Here we rename the `build` CI step to `build-docs` and we test a dev build of the docs, which should be way faster.

* Let Vercel handle testing docs + don't run JS tests on docs changes.

.github/workflows/ci.yml

@@ -7,6 +7,10 @@ on:
   push:
     branches: ["main"]
   pull_request:
+    # Do not run this workflow if only docs changed.
+    paths-ignore:
+      - 'docs/**'
+  workflow_dispatch:  # Allows triggering the workflow manually in GitHub UI
 
 
 # If another push to the same PR or branch happens while this workflow is still running,
@@ -50,24 +54,6 @@ jobs:
       - name: Check linting
         run: yarn run lint
 
-  build:
-    name: Build and check types
-    runs-on: ubuntu-latest
-    env:
-      PUPPETEER_SKIP_DOWNLOAD: "true"
-      PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD: "true"
-    steps:
-      - uses: actions/checkout@v3
-      - name: Use Node.js 18.x
-        uses: actions/setup-node@v3
-        with:
-          node-version: 18.x
-          cache: "yarn"
-      - name: Install dependencies
-        run: yarn install --immutable
-      - name: Build and check types
-        run: yarn run build
-
   test:
     name: Unit Tests
     strategy: