documentation

Pull request #630: Feature/ZBXNEXT-7469 6.4 D

Merge in WEB/documentation from feature/ZBXNEXT-7469-6.4-D to release/6.4

* commit 'dc83168f11e8d5a473f0d778c9794ce38c7f266d': (22 commits)
  [ZBXNEXT-7469] numbering fixes
  [ZBXNEXT-7469] updated some formatting
  [ZBXNEXT-7469] more formatting fixes
  [ZBXNEXT-7469] updated Dev center documentation
  removed placeholders from plugin section
  [ZBXNEXT-7469] added how_to for modules (not widget)
  [ZBXNEXT-7469] added formatting changes to manifest.json
  [ZBXNEXT-7469] updated formatting
  [ZBXNEXT-7469] updated manifest.json
  [ZBXNEXT-7469] updated widget how to
  [ZBXNEXT-7469] updated widget how_to
  [ZBXNEXT-7469] updated widget documentation
  [ZBXNEXT-7469] updated manifest.json formatting and how_to.md
  [ZBXNEXT_7469] updated code formatting
  [ZBXNEXT-7469] updated widget how_to.md
  [ZBXNEXT-7469] updated all tab to spaces
  [ZBXNEXT-7469] fixed module demo archive; fixed paths to file for code sources
  [ZBXNEXT-7469] Dev draft of widget tutorial
  [ZBXNEXT-7469] added widget documentation layout
  [ZBXNEXT-7469] updated dummy Dev section structure
  ...