Build a website. (#4189)

Demo site: https://jonmeow.carbon-lang.dev/

I'm trying to keep work under the `/website` subdirectory so that the
misc files don't interfere with unrelated views of the repository. The
`prebuild.py` script does some work to move things around and add
frontmatter, helping the jekyll generation.

I'm using the "just-the-docs" theme because I think it's a decent match
for what we want, and getting jekyll up and running with it wasn't too
difficult. Note #1526 proposed using Docusaurus; I started out there,
but was having trouble getting it working with newer versions. The
plugins in particular I got stuck trying to make work, which sent me
looking for options that we could have working with less customization.
I do lean towards jekyll though, because it's what GH uses so hopefully
we can get a more consistent experience.

Having a website has been approved for a while under #1492, but hasn't
been a priority. I'm mainly doing this because I want to just be able to
point people to carbon-lang.dev and have easy links that way.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

.github/workflows/gh_pages_ci.yaml

@@ -0,0 +1,41 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: GitHub Pages CI
+
+on:
+  pull_request:
+
+# Cancel previous workflows on the PR when there are multiple fast commits.
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  # Build job
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+        with:
+          egress-policy: audit
+
+      - name: Checkout
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+      - name: Prebuild actions
+        run: ./website/prebuild.py
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@a6e6f86333f0a2523ece813039b8b4be04560854 # v1.190.0
+        with:
+          # Runs 'bundle install' and caches installed gems automatically.
+          bundler-cache: true
+          # Increment this number if you need to re-download cached gems.
+          cache-version: 0
+      - name: Build with Jekyll
+        run: bundle exec jekyll build

.github/workflows/gh_pages_deploy.yaml

@@ -0,0 +1,76 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: GitHub Pages deploy
+
+on:
+  # Runs on pushes targeting the default branch.
+  push:
+    branches: ['trunk']
+
+  # Allows you to run this workflow manually from the Actions tab.
+  workflow_dispatch:
+
+# Cancel previous workflows on the PR when there are multiple fast commits.
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
+# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages.
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+        with:
+          egress-policy: audit
+
+      - name: Checkout
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+      - name: Prebuild actions
+        run: ./website/prebuild.py
+      - name: Setup Pages
+        uses: actions/configure-pages@983d7736d9b0ae728b81ab479565c72886d7745b # v5.0.0
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@a6e6f86333f0a2523ece813039b8b4be04560854 # v1.190.0
+        with:
+          # Runs 'bundle install' and caches installed gems automatically.
+          bundler-cache: true
+          # Increment this number if you need to re-download cached gems.
+          cache-version: 0
+      - name: Build with Jekyll
+        env:
+          JEKYLL_ENV: production
+        run: |
+          bundle exec jekyll build --verbose \
+            --source ./ \
+            --destination ./_site \
+            --baseurl "${{ steps.pages.outputs.base_path }}"
+      - name: Upload artifact
+        # Automatically uploads an artifact from the './_site' directory by
+        # default.
+        uses: actions/upload-pages-artifact@56afc609e74202658d3ffba0e8f6dda462b719fa # v3.0.1
+
+  deploy:
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+      - name: Harden Runner
+        uses: step-security/harden-runner@5c7944e73c4c2a096b17a9cb74d65b6c2bbafbde # v2.9.1
+        with:
+          egress-policy: audit
+
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@d6db90164ac5ed86f2b6aed7e0febac5b3c0c03e # v4.0.5