documentation

Pull request #777: feature/ZBXNEXT-8087-6.4-D2 (Restored documentation on template nesting).

Merge in WEB/documentation from feature/ZBXNEXT-8087-6.4-D2 to release/6.4

* commit 'a3f7df3fcbbda1a4195c5379313ac6468291d3da':
  [ZBXNEXT-8087] Updated the restored documentation on template nesting
  [ZBXNEXT-8087] Updated the restored documentation on template nesting
  [ZBXNEXT-8087] Restored documentation on template nesting