documentation

[ZBXNEXT-7668]-6.2-D SLA documentation update

Merge in WEB/documentation from feature/ZBXNEXT-7668-6.2-D to release/6.2

* commit '30f71eb1795d701a1ddcf05e1174bbd1557304e8':
  One more modification suggested in ZBX-21007
  final fixes
  upgrade procedure description
  Updated user role permissions
  added some more changes
  sla.md improvements
  added SLA details
  updated frontend - service actions
  added SLA report images to example.md
  more updates
  Adding examples.md
  sla updates part 1
  updated service tree page
  more primary page improvements
  rewriting front page
  improving Servvices section structure