fix(ci): complete PR preview path offset and fork PR handling (#6667)

* fix(ci): add path offset for PR preview subdirectory baseURL

When PR preview builds use a subdirectory baseURL like
/docs-v2/pr-preview/pr-XXXX/, shortcodes that parse .RelPermalink
to detect product context fail because the path has extra segments.

This fix:
- Adds config/pr-preview/params.yml with prPreviewPathOffset: 3
- Updates workflow to use -e pr-preview environment
- Updates api-endpoint, influxdb/host, and children shortcodes to
  use the offset when indexing path segments
- Adds nil-safety with default fallback for placeholder_host

Normal builds are unaffected (offset defaults to 0).

* fix(ci): add path offset to product-name and sidebar for PR previews

Apply the same prPreviewPathOffset fix to product-name.html and
sidebar.html that was applied in the initial PR #6665.

These templates parse RelPermalink to detect product context, but when
baseURL includes a subdirectory path (e.g., /docs-v2/pr-preview/pr-XXXX/),
the path indices shift. This fix uses the configurable offset to skip
extra path segments in PR preview builds.

* fix(ci): skip PR preview for fork PRs and add notice comment

Fork PRs cannot deploy to gh-pages because GITHUB_TOKEN has read-only
access to the base repository. This is a GitHub security feature.

Changes:
- Add condition to skip preview job for fork PRs
- Add fork-notice job to post helpful comment explaining limitation
- Include local preview instructions for contributors

Author: jstirnaman

.github/workflows/pr-preview.yml

@@ -35,12 +35,45 @@ jobs:
             echo "should-run=true" >> $GITHUB_OUTPUT
           fi
 
+  # Notify fork PRs that preview is not available
+  fork-notice:
+    needs: check-draft
+    if: |
+      needs.check-draft.outputs.should-run == 'true' &&
+      github.event.action != 'closed' &&
+      github.event.pull_request.head.repo.full_name != github.repository
+    runs-on: ubuntu-latest
+    steps:
+      - name: Post fork notice comment
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const { data: comments } = await github.rest.issues.listComments({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: context.issue.number
+            });
+
+            const marker = '<!-- pr-preview-fork-notice -->';
+            const existing = comments.find(c => c.body.includes(marker));
+
+            if (!existing) {
+              await github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: context.issue.number,
+                body: `${marker}\n## 📝 PR Preview Not Available\n\nPR previews are not available for pull requests from forks due to GitHub Actions security restrictions.\n\nTo preview your changes locally, run:\n\`\`\`bash\nnpx hugo server\n\`\`\`\n\nOnce merged, your changes will be visible on the production site.`
+              });
+            }
+
   # Build and deploy preview
   preview:
     needs: check-draft
+    # Skip fork PRs - GITHUB_TOKEN doesn't have write access to push to gh-pages
     if: |
       needs.check-draft.outputs.should-run == 'true' &&
-      github.event.action != 'closed'
+      github.event.action != 'closed' &&
+      github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest
     steps:
       - name: Checkout

layouts/partials/sidebar.html

@@ -1,8 +1,10 @@
 {{ $currentPage := . }}
 {{ $productPathData := findRE "[^/]+.*?" .RelPermalink }}
-{{ $product := index $productPathData 0 }}
+{{/* Support deployments (such as CI tools) with subdirectory baseURL */}}
+{{ $pathOffset := .Site.Params.prPreviewPathOffset | default 0 }}
+{{ $product := index $productPathData (add $pathOffset 0) }}
 {{ $productName := (index .Site.Data.products $product).name }}
-{{ $currentVersion := index $productPathData 1 }}
+{{ $currentVersion := index $productPathData (add $pathOffset 1) }}
 
 <!-- Menu Key -->
 {{ .Scratch.Set "menuKey" "menu"}}

layouts/shortcodes/product-name.html

@@ -1,5 +1,7 @@
 {{- $productPathData := findRE "[^/]+.*?" .Page.RelPermalink -}}
-{{- $currentProduct := index $productPathData 1 -}}
+{{- /* Support deployments (such as CI tools) with subdirectory baseURL */ -}}
+{{- $pathOffset := .Site.Params.prPreviewPathOffset | default 0 -}}
+{{- $currentProduct := index $productPathData (add $pathOffset 1) -}}
 {{- $length := .Get 0 | default "long" -}}
 {{- $omit := .Get "omit" | default "" -}}
 {{- $scratch := newScratch -}}