Add multi-version pkgdown documentation with version dropdown menu

.github/MULTI_VERSION_DOCS.md

@@ -0,0 +1,153 @@
+# Multi-Version Documentation Setup
+
+This repository uses multi-version pkgdown documentation with a version dropdown menu, based on the [insightsengineering/tern](https://github.com/insightsengineering/tern) setup.
+
+## Features
+
+### Version Dropdown Menu
+Users can switch between different versions of the documentation using a dropdown menu in the navbar:
+- **main**: Development version (latest commits on main branch)
+- **latest-tag**: Most recent release version
+- **v*.*.*** Version-tagged releases (e.g., v1.0.0, v1.1.0)
+
+### Default Landing Page
+The default page at https://ucd-serg.github.io/serodynamics/ shows the **latest-tag** (most recent release) documentation.
+
+### PR Previews
+Pull requests automatically generate preview documentation at:
+- `https://ucd-serg.github.io/serodynamics/preview/pr<number>/`
+- Preview link is posted as a comment on the PR
+- Previews are automatically cleaned up when PRs are closed
+
+## How It Works
+
+### Workflow: `.github/workflows/docs.yaml`
+
+The workflow has four jobs:
+
+1. **docs**: Builds the pkgdown site
+   - Runs on: PRs, pushes to main, version tags
+   - Deploys to version-specific directories on gh-pages
+
+2. **multi-version-docs**: Creates version dropdown
+   - Runs on: pushes to main, workflow_dispatch
+   - Uses `insightsengineering/r-pkgdown-multiversion@v3`
+   - Generates navigation between versions
+
+3. **upload-release-assets**: Upload pkgdown.zip
+   - Runs on: version tags only
+   - Uploads documentation to GitHub Releases
+
+4. **cleanup-pr-preview**: Clean up PR previews
+   - Runs on: PR close events
+   - Removes preview directories from gh-pages
+
+### Configuration
+
+#### `pkgdown/_pkgdown.yml`
+```yaml
+template:
+  bootstrap: 5  # Required for multi-version action
+
+url: https://ucd-serg.github.io/serodynamics/  # Required
+```
+
+#### `DESCRIPTION`
+```
+URL: https://github.com/UCD-SERG/serodynamics, 
+     https://ucd-serg.github.io/serodynamics/
+```
+
+### Version Filtering
+
+Versions shown in the dropdown are controlled by:
+
+**refs-order**: `"main latest-tag"`
+- Determines the order of versions in the dropdown
+
+**branches-or-tags-to-list**: `'^main$|^latest-tag$|^v([0-9]+\\.)?([0-9]+\\.)?([0-9]+)$'`
+- Regex pattern matching versions to include
+- Matches: `main`, `latest-tag`, and semver tags like `v1.0.0`
+
+## Creating a New Version
+
+To create a new versioned documentation:
+
+1. **Tag a release**:
+   ```bash
+   git tag v1.0.0
+   git push origin v1.0.0
+   ```
+
+2. **Workflow runs automatically**:
+   - Builds pkgdown site for v1.0.0
+   - Deploys to `gh-pages/v1.0.0/`
+   - Updates version dropdown to include v1.0.0
+   - Uploads pkgdown.zip to the GitHub release
+
+3. **Version appears in dropdown**:
+   - Users can now select v1.0.0 from the versions menu
+   - If it's the latest tag, it becomes the default landing page
+
+## Triggering Builds
+
+### Automatic Triggers
+- **PR opened/updated**: Builds preview documentation
+- **Push to main**: Rebuilds main version docs
+- **Version tag pushed**: Builds new version docs
+- **PR closed**: Cleans up preview
+
+### Manual Trigger
+Use the Actions tab to manually trigger the workflow via `workflow_dispatch`.
+
+### Path Filtering
+The workflow only runs when these files change:
+- `inst/templates/**`
+- `pkgdown/**`
+- `_pkgdown.*`
+- `DESCRIPTION`
+- `**.md`, `**.Rmd`
+- `man/**`
+- `LICENSE.*`
+- `NAMESPACE`
+
+## Troubleshooting
+
+### Workflow requires approval
+First-time workflows using external actions need manual approval:
+1. Go to repository Actions tab
+2. Find the workflow run
+3. Click "Approve and run"
+4. Subsequent runs will execute automatically
+
+### Version not appearing in dropdown
+Check that:
+1. Documentation was successfully built for that version
+2. Version name matches the regex pattern
+3. Version is listed in `refs-order` or matches `branches-or-tags-to-list`
+
+### Default landing page not updating
+Ensure:
+1. `default-landing-page: "latest-tag"` is set in workflow
+2. Latest tag has documentation built
+3. Multi-version-docs job completed successfully
+
+## Comparison to Original Setup
+
+### Before (pkgdown.yaml)
+- Single version (main branch only)
+- PR previews only
+- No version switching
+
+### After (docs.yaml)
+- Multiple versions (main + all releases)
+- Version dropdown menu
+- PR previews maintained
+- Default to latest release
+- Version-specific documentation URLs
+
+## Resources
+
+- [insightsengineering/r-pkgdown-multiversion](https://github.com/insightsengineering/r-pkgdown-multiversion)
+- [insightsengineering/tern](https://github.com/insightsengineering/tern) (reference implementation)
+- [pkgdown documentation](https://pkgdown.r-lib.org/)