Doc: fix http->https, remove/update broken links

Broken links in Admin Guide:

• wincron original site redirecting, and not able to find original
  product (search yields multiple softwares named wincron, all fairly
  old and seemingly unmaintained) -> removing reference
• mysqlhotcopy is no longer available since MySQL 5.7 - replace with
  mysqldump
• added link to Percona XtraBackup