Create GitHub Actions workflow for deploying docs.

This allows+requires us to [replace each release's Javadoc directory with a Javadoc archive file](407e30a).

As noted in that commit, this saves a ton of time when switching branches: I'm now seeing ~3s instead of ~30s to switch to `gh-pages` and ~2s instead of ~7s to switch back to `master`. (It probably slows down downloading the entire repo's history, though, since the repo history now contains all the original files *plus* the archives that contain them all. I can fix that if I ever rewrite the whole history of `gh-pages`.)

I'd raised this possibility back in #7597 (comment).

To make this take effect, I'll need to update https://github.com/google/guava/settings/pages to use a Source of "GitHub Actions" instead of "Deploy from a branch." This matches what we already do for JSpecify.

I notice that it may be a little weird for our `master` branch to have a GitHub Actions setup that pushes to our `gh-pages` branch, which then has a GitHub Actions setup that deploys that branch. I suspect that it at least makes sense to have two separate actions, since we sometimes push to the `gh-pages` branch outside the context of a push to `main`—notably, when we perform a release.

(One thing that we could consider is the idea of merging `gh-pages` into `master`, presumably under a `docs` directory. But I think that's been possible forever, and we haven't done it. That's probably for the best, since it would make a shallow clone of `master` slow because of all the Javadoc.)

RELNOTES=n/a
PiperOrigin-RevId: 851409347

Author: cpovirk

.github/workflows/docs.yml

@@ -0,0 +1,56 @@
+name: Deploy docs
+
+on:
+  push:
+    branches:
+      - gh-pages
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+concurrency:
+  group: "pages"
+
+jobs:
+  build:
+    name: Build GitHub Pages site
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8
+
+      - name: Extract Javadoc archives
+        working-directory: releases
+        run: |
+          for file in *.tar.gz; do
+            tar -xzf "$file"
+            rm "$file"
+          done
+
+      - name: Setup Pages
+        uses: actions/configure-pages@983d7736d9b0ae728b81ab479565c72886d7745b
+
+      - name: Build with Jekyll
+        uses: actions/jekyll-build-pages@44a6e6beabd48582f863aeeb6cb2151cc1716697
+        with:
+          source: ./
+          destination: ./_site
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@7b1f4a764d45c48632c6b24a0339c27f5614fb0b
+        with:
+          path: ./_site
+
+  deploy:
+    name: Deploy to GitHub Pages
+    needs: build
+    runs-on: ubuntu-latest
+    environment:
+      name: github-pages
+      url: ${{ steps.deployment.outputs.page_url }}
+    steps:
+      - name: Deploy to GitHub Pages
+        id: deployment
+        uses: actions/deploy-pages@d6db90164ac5ed86f2b6aed7e0febac5b3c0c03e