CA-108875: Update XenCert doc for Clearwater

- Documented that XenCert is now a supplemental pack.
- Increased XenServer and XenCert version numbers.
- Improved a few wordings.
- Renamed docx files to not contain the version to allow tracking beyond
  multiple versions.
- Updated headers with the most recent Citrix Ready logo to avoid regressing.
- Droped ancient readme that did not apply anymore.