documentation

Pull request #383: ZBXNEXT-6181 6.4  Updated proxy upgrading features.

Merge in WEB/documentation from feature/ZBXNEXT-6181-6.4-D to release/6.4

* commit 'ec3459d0525093adeb5f213062b6bc8a312c4ab4':
  ZBXNEXT-6181 Edited Whatsnew page.
  ZBXNEXT-6181 Edited Upgrade notes page.
  ZBXNEXT-6181 Edited Upgrade  page.
  ZBXNEXT-6181 Edited Upgrade and Whatsnew pages.
  ZBXNEXT-6181 Edited Upgrade procedure.
  ZBXNEXT-6181 Edited Upgrade notes/Upgrade proced.
  ZBXNEXT-6181 Deleted a paragraph in Debian/Ubuntu.
  ZBXNEXT-6181 Updated Whatsnew page.
  ZBXNEXT-6181 Updated Upgrade notes page.
  ZBXNEXT-6181 Updated Whatsnew page.
  ZBXNEXT-6181 Updated Debian/Ubuntu page.
  ZBXNEXT-6181 Updated Upgrade procedure page.