documentation

Pull request #2799: feature/ZBX-21543-7.3-D-v2. added "Checking for not supported value" example.

Merge in WEB/documentation from feature/ZBX-21543-7.3-D-v2 to release/7.4

* commit '397f61146d56c15dadcca839f4c07eacb377efa1':
  [ZBX-21543] common user pain point made explicit
  [ZBX-21543] added "Checking for not supported value" example
  updated "Preprocessing examples" page according to Documentation guidelines