chore(pkgdown): wire GH Actions deploy to gh-pages (#15)

* Add _pkgdown.yml with site URL (https://vincentguyader.github.io/shiny2docker/)
  and bootstrap 5 template.
* Add .github/workflows/pkgdown.yaml: build on push to main / PR / release,
  deploy to gh-pages via JamesIves/github-pages-deploy-action.
* Rbuildignore: exclude _pkgdown.yml, docs/, pkgdown/.
* gitignore: exclude /docs/ (generated, deployed via Action, not committed).

After first run on main, enable GitHub Pages in repo settings:
Source = "Deploy from a branch", Branch = gh-pages, Folder = /.

Author: VincentGuyader

.Rbuildignore

@@ -11,3 +11,6 @@
 ^CRAN-SUBMISSION$
 ^doc$
 ^Meta$
+^_pkgdown\.yml$
+^docs$
+^pkgdown$

.github/workflows/pkgdown.yaml

@@ -0,0 +1,44 @@
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+  release:
+    types: [published]
+  workflow_dispatch:
+
+name: pkgdown.yaml
+
+permissions: read-all
+
+jobs:
+  pkgdown:
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: r-lib/actions/setup-pandoc@v2
+
+      - uses: r-lib/actions/setup-r@v2
+        with:
+          use-public-rspm: true
+
+      - uses: r-lib/actions/setup-r-dependencies@v2
+        with:
+          extra-packages: any::pkgdown, local::.
+          needs: website
+
+      - name: Build site
+        run: pkgdown::build_site_github_pages(new_process = FALSE, install = FALSE)
+        shell: Rscript {0}
+
+      - name: Deploy to GitHub pages
+        if: github.event_name != 'pull_request'
+        uses: JamesIves/github-pages-deploy-action@v4.5.0
+        with:
+          clean: false
+          branch: gh-pages
+          folder: docs

.gitignore

@@ -5,3 +5,4 @@
 inst/doc
 /doc/
 /Meta/
+/docs/

_pkgdown.yml

@@ -0,0 +1,4 @@
+url: https://vincentguyader.github.io/shiny2docker/
+
+template:
+  bootstrap: 5