fix(ci): add path offset for PR preview subdirectory baseURL (#6665)

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).

Author: jstirnaman