documentation

Pull request #356: Feature/ZBXNEXT-2043 6.4 D

Merge in WEB/documentation from feature/ZBXNEXT-2043-6.4-D to release/6.4

* commit 'd95a1ebde78e19a8f1fe80b8b4d4db360d3b6ad6':
  [ZBXNEXT-2043] updated Custom performance counters
  [ZBXNEXT-2043] perf_counters.md edited title
  [ZBXNEXT-2043] added some imporvements to performance counters page
  [ZBXNEXT-2043] Added page for custom performance counters